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/services/backup/mysql-backup.nix

@@ -37,12 +37,12 @@ in
 
     services.mysqlBackup = {
 
-      enable = mkEnableOption (lib.mdDoc "MySQL backups");
+      enable = mkEnableOption "MySQL backups";
 
       calendar = mkOption {
         type = types.str;
         default = "01:15:00";
-        description = lib.mdDoc ''
+        description = ''
           Configured when to run the backup service systemd unit (DayOfWeek Year-Month-Day Hour:Minute:Second).
         '';
       };
@@ -50,7 +50,7 @@ in
       user = mkOption {
         type = types.str;
         default = defaultUser;
-        description = lib.mdDoc ''
+        description = ''
           User to be used to perform backup.
         '';
       };
@@ -58,7 +58,7 @@ in
       databases = mkOption {
         default = [];
         type = types.listOf types.str;
-        description = lib.mdDoc ''
+        description = ''
           List of database names to dump.
         '';
       };
@@ -66,7 +66,7 @@ in
       location = mkOption {
         type = types.path;
         default = "/var/backup/mysql";
-        description = lib.mdDoc ''
+        description = ''
           Location to put the gzipped MySQL database dumps.
         '';
       };
@@ -74,7 +74,7 @@ in
       singleTransaction = mkOption {
         default = false;
         type = types.bool;
-        description = lib.mdDoc ''
+        description = ''
           Whether to create database dump in a single transaction
         '';
       };