nixos: remove redundant services.dovecot2.package option

Instead of using this option, please modify the dovecot package by means of an
override. For example:

nixpkgs.config.packageOverrides = super: {
dovecot = super.dovecot.override { withPgSQL = true; };
};

Closes https://github.com/NixOS/nixpkgs/issues/14097.

nixos/modules/rename.nix

···
     (mkRemovedOptionModule [ "services" "printing" "cupsdConf" ])
     (mkRemovedOptionModule [ "services" "xserver" "startGnuPGAgent" ])
     (mkRemovedOptionModule [ "services" "phpfpm" "phpIni" ])
+    (mkRemovedOptionModule [ "services" "dovecot2" "package" ])
 
   ];
 }

nixos/modules/services/mail/dovecot.nix

···
 
 let
   cfg = config.services.dovecot2;
-  dovecotPkg = cfg.package;
+  dovecotPkg = pkgs.dovecot;
 
   baseDir = "/run/dovecot2";
   stateDir = "/var/lib/dovecot";
···
       description = "Additional listeners to start when Dovecot is enabled.";
     };
 
-    package = mkOption {
-      type = types.package;
-      default = pkgs.dovecot;
-      defaultText = "pkgs.dovecot";
-      description = "Dovecot package to use.";
-    };
-
     user = mkOption {
       type = types.str;
       default = "dovecot2";
···
       description = ''
         Symlinks the contents of lib/dovecot of every given package into
         /etc/dovecot/modules. This will make the given modules available
-        if a dovecot package with the module_dir patch applied (like
-        pkgs.dovecot22, the default) is being used.
+        if a dovecot package with the module_dir patch applied is being used.
       '';
     };