security: do not check subscriptions on "internal.ballot" event

The event is used by appliers as a better alternative to IPROTO_VOTE.
Besides, event subscribers receive exactly the same payload as the ones
sending IPROTO_VOTE. So there's no need to guard against subscription to
this particular event as long as IPROTO_VOTE isn't guarded.

Follow-up #5272

NO_DOC=no user-visible changes
NO_CHANGELOG=no user-visible changes
NO_TEST=tested by ee