pf: Fully remove interrupt events on vnet cleanup
swi_remove() removes the software interrupt handler but does not remove
the associated interrupt event.
This is visible when creating and remove a vnet jail in `procstat -t
We can remove it manually with intr_event_destroy().
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D29211
(cherry picked from commit cecfaf9bede9665d6a10f1e575cd5d575450cff7)