nixos/pam: remove explicit hardcoded yescrypt

Both `pam_unix.so` and `pam_unix_ng.so` look at `ENCRYPT_METHOD` in
`/etc/login.defs` to determine the algorithm to use for password
encryption: https://github.com/thkukuk/account-utils/blob/66fbd0382be49d99cb70410a4696c796684ffbf8/src/pam_unix_ng-common.c#L27-L62
If this is not set, both already default to `YESCRYPT`.
The shadow module makes this configurable via
`security.loginDefs.settings.ENCRYPT_METHOD`, which also defaults to `YESCRYPT`.
Seeing as what was previously hardcoded is default anyways, with a global
configuration option to change it, there is no point to keep this.

nixos/modules/security/pam.nix

@@ -1422,7 +1422,6 @@ let
                 modulePath = config.security.pam.pam_unixModulePath;
                 settings = {
                   nullok = true;
-                  yescrypt = lib.mkIf config.security.pam.enableLegacySettings true;
                 };
               }
               {

nixos/modules/services/display-managers/gdm.nix

@@ -577,7 +577,6 @@ in
               control = "requisite";
               modulePath = "${config.security.pam.package}/lib/security/pam_unix.so";
               settings.nullok = true;
-              settings.yescrypt = true;
             }
           ];
 

nixos/modules/services/x11/display-managers/lightdm.nix

@@ -456,7 +456,6 @@ in
             control = "requisite";
             modulePath = "${config.security.pam.package}/lib/security/pam_unix.so";
             settings.nullok = true;
-            settings.yescrypt = true;
           }
         ];