Revert "postgresql: version the dataDir by default"

This reverts commit 7f1e728606.

This would have been nice if we had had it from the start, but now it
just breaks things for existing users. Maybe we can add it conditionally
when new postgres versions come out.

Signed-off-by: Shea Levy <shea@shealevy.com>

modules/services/databases/postgresql.nix

@@ -74,7 +74,7 @@ in
       };
 
       dataDir = mkOption {
-        default = "/var/db/postgresql-${cfg.package.psqlSchema}";
+        default = "/var/db/postgresql";
         description = ''
           Data directory for PostgreSQL.
         '';