intr: Remove dead code from intr_event_remove_handler()

We currently destroy the ithread in intr_event_destroy(). In
preparation for fixing a bug there, remove this dead code and reorganize
a bit to avoid some code duplication. No functional change intended.

Reviewed by: kib, jhb
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D45490