Refactor: fencer: Drop notifications for topology level add/remove

These were never hooked up. No internal client tries to register for
these notifications. If an external client tried (via
stonith_api_operations_t:register_notification), it would be a no-op.
fenced_parse_notify_flag() (previously get_stonith_flag()) returns the 0
flag for these notification type strings. So the "client gets notified
for this type of event" bit could never get set.

We could fix it, since some amount of work was done to set this up.
However, if nothing internal needs it, I figure we might as well drop
it.

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