remove Diffie Hillman parameter creation as it is handled by the upstream dovecot2 module as of 18.09

default.nix

@@ -405,17 +405,6 @@ in
       '';
     };
 
-    dhParamBitLength = mkOption {
-      type = types.int;
-      default = 2048;
-      description =
-        ''
-        Length of the Diffie Hillman prime used (in bits). It might be a good
-        idea to set this to 4096 for security purposed, but it will take a _very_
-        long time to create this prime on startup.
-        '';
-    };
-
     debug = mkOption {
       type = types.bool;
       default = false;