nixos: remove all uses of lib.mdDoc

these changes were generated with nixq 0.0.2, by running

  nixq ">> lib.mdDoc[remove] Argument[keep]" --batchmode nixos/**.nix
  nixq ">> mdDoc[remove] Argument[keep]" --batchmode nixos/**.nix
  nixq ">> Inherit >> mdDoc[remove]" --batchmode nixos/**.nix

two mentions of the mdDoc function remain in nixos/, both of which
are inside of comments.

Since lib.mdDoc is already defined as just id, this commit is a no-op as
far as Nix (and the built manual) is concerned.

nixos/modules/security/duosec.nix

@@ -36,24 +36,24 @@ in
       ssh.enable = mkOption {
         type = types.bool;
         default = false;
-        description = lib.mdDoc "If enabled, protect SSH logins with Duo Security.";
+        description = "If enabled, protect SSH logins with Duo Security.";
       };
 
       pam.enable = mkOption {
         type = types.bool;
         default = false;
-        description = lib.mdDoc "If enabled, protect logins with Duo Security using PAM support.";
+        description = "If enabled, protect logins with Duo Security using PAM support.";
       };
 
       integrationKey = mkOption {
         type = types.str;
-        description = lib.mdDoc "Integration key.";
+        description = "Integration key.";
       };
 
       secretKeyFile = mkOption {
         type = types.nullOr types.path;
         default = null;
-        description = lib.mdDoc ''
+        description = ''
           A file containing your secret key. The security of your Duo application is tied to the security of your secret key.
         '';
         example = "/run/keys/duo-skey";
@@ -61,14 +61,14 @@ in
 
       host = mkOption {
         type = types.str;
-        description = lib.mdDoc "Duo API hostname.";
+        description = "Duo API hostname.";
       };
 
       groups = mkOption {
         type = types.str;
         default = "";
         example = "users,!wheel,!*admin guests";
-        description = lib.mdDoc ''
+        description = ''
           If specified, Duo authentication is required only for users
           whose primary group or supplementary group list matches one
           of the space-separated pattern lists. Refer to
@@ -79,7 +79,7 @@ in
       failmode = mkOption {
         type = types.enum [ "safe" "secure" ];
         default = "safe";
-        description = lib.mdDoc ''
+        description = ''
           On service or configuration errors that prevent Duo
           authentication, fail "safe" (allow access) or "secure" (deny
           access). The default is "safe".
@@ -89,7 +89,7 @@ in
       pushinfo = mkOption {
         type = types.bool;
         default = false;
-        description = lib.mdDoc ''
+        description = ''
           Include information such as the command to be executed in
           the Duo Push message.
         '';
@@ -98,7 +98,7 @@ in
       autopush = mkOption {
         type = types.bool;
         default = false;
-        description = lib.mdDoc ''
+        description = ''
           If `true`, Duo Unix will automatically send
           a push login request to the user’s phone, falling back on a
           phone call if push is unavailable. If
@@ -112,7 +112,7 @@ in
       motd = mkOption {
         type = types.bool;
         default = false;
-        description = lib.mdDoc ''
+        description = ''
           Print the contents of `/etc/motd` to screen
           after a successful login.
         '';
@@ -121,7 +121,7 @@ in
       prompts = mkOption {
         type = types.enum [ 1 2 3 ];
         default = 3;
-        description = lib.mdDoc ''
+        description = ''
           If a user fails to authenticate with a second factor, Duo
           Unix will prompt the user to authenticate again. This option
           sets the maximum number of prompts that Duo Unix will
@@ -142,7 +142,7 @@ in
       acceptEnvFactor = mkOption {
         type = types.bool;
         default = false;
-        description = lib.mdDoc ''
+        description = ''
           Look for factor selection or passcode in the
           `$DUO_PASSCODE` environment variable before
           prompting the user for input.
@@ -157,7 +157,7 @@ in
       fallbackLocalIP = mkOption {
         type = types.bool;
         default = false;
-        description = lib.mdDoc ''
+        description = ''
           Duo Unix reports the IP address of the authorizing user, for
           the purposes of authorization and whitelisting. If Duo Unix
           cannot detect the IP address of the client, setting
@@ -173,7 +173,7 @@ in
       allowTcpForwarding = mkOption {
         type = types.bool;
         default = false;
-        description = lib.mdDoc ''
+        description = ''
           By default, when SSH forwarding, enabling Duo Security will
           disable TCP forwarding. By enabling this, you potentially
           undermine some of the SSH based login security. Note this is