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
12`.

We can remove it manually with intr_event_destroy().

PR: 254171
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D29211