Refactor: fencer: Drop redundant topology add/remove helpers

The logic is now simple enough not to merit separate functions. All we
have to do is allow fenced_register_level() and
fenced_unregister_level() to accept a NULL result argument.
pcmk__set_result() and related functions accept a NULL argument.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>