Refactor: based: Defunctionize do_cib_notify()

It's only used with cib_diff_notify(). The only other analogous
notifier, cib_replace_notify(), doesn't use it.

As of right now, the logic is sufficiently short yet sufficiently
different in both functions that I don't think it's worth adapting
cib_replace_notify() to use a functionized do_cib_notify().

It's also worth noting that the logic is simplified after pulling it
into cib_diff_notify(): we can be sure that diff (previously
result_data) is non-NULL, and we don't have to worry about the message
type.

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