nixos/postgresql: remove postgresql_11-backup-all test

I don't really understand why this is only for a single version, so I
figured I'd just add this to the test matrix to also cover this
test-case for each major. Now, there's also one thing less to take care of
when removing old postgresql versions.

nixos/tests/postgresql.nix

@@ -219,8 +219,6 @@ let
 in
   concatMapAttrs (name: package: {
     ${name} = make-postgresql-test name package false;
+    ${name + "-backup-all"} = make-postgresql-test "${name + "-backup-all"}" package true;
     ${name + "-clauses"} = mk-ensure-clauses-test name package;
   }) postgresql-versions
-  // {
-    postgresql_11-backup-all = make-postgresql-test "postgresql_11-backup-all" postgresql-versions.postgresql_11 true;
-  }