postgresql: remove version from passthru

It is already available in the regular attrs.

`passthru.version` prevents `overrideAttrs (attrs: { version = "x"; })` from
taking effect.

pkgs/servers/sql/postgresql/default.nix

@@ -125,7 +125,7 @@ let
     disallowedReferences = [ stdenv.cc ];
 
     passthru = {
-      inherit readline psqlSchema version;
+      inherit readline psqlSchema;
 
       pkgs = let
         scope = { postgresql = this; };