applier: pass applier as event to ballot update triggers

Now that the event isn't used to pass the success status, we may pass
applier there for consistency with on_state triggers.

Besides, this allows to simplify applier_ballot_data struct, which
doesn't need to store applier anymore.

In-scope-of #8509

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring