Revert "postgresql: properly fix permissions issue by in postStart"

Reverting postgres superuser changes until after stable.

This reverts commit c66be6378d.

nixos/modules/services/databases/postgresql.nix

@@ -225,14 +225,14 @@ in
         # Wait for PostgreSQL to be ready to accept connections.
         postStart =
           ''
-            while ! su -s ${pkgs.stdenv.shell} postgres -c 'psql postgres -c ""' 2> /dev/null; do
+            while ! psql postgres -c "" 2> /dev/null; do
                 if ! kill -0 "$MAINPID"; then exit 1; fi
                 sleep 0.1
             done
 
             if test -e "${cfg.dataDir}/.first_startup"; then
               ${optionalString (cfg.initialScript != null) ''
-                cat "${cfg.initialScript}" | su -s ${pkgs.stdenv.shell} postgres -c 'psql postgres'
+                cat "${cfg.initialScript}" | psql postgres
               ''}
               rm -f "${cfg.dataDir}/.first_startup"
             fi