Revert "nixos/acme: Fix allowKeysForGroup not applying immediately"

This reverts commit 5532065d.

As far as I can tell setting RemainAfterExit=true here completely breaks
certificate renewal, which is really bad!

the sytemd timer will activate the service unit every OnCalendar=,
however with RemainAfterExit=true the service is already active! So the
timer doesn't rerun the service!

The commit also broke the actual tests, (As it broke activation too)
but this was fixed later in https://github.com/NixOS/nixpkgs/pull/76052
I wrongly assumed that PR fixed renewal too, which it didn't!

testing renewals is hard, as we need to sleep in tests.