pctrie: iter_remove check from panic to KASSERT

pctrie_iter_remove checks to see if the thing the iterator points to
is actually there, and panics if it is not. This panic would likely
indicate the same iterator had been used for removal twice, without
advancing the iterator in-between. This test takes a bit of time, and
as it indicates a programmer error rather than some external
condition, it is better handled as a KASSERT. This means with KASSERTs
disabled, a wee bit of time is saved.

Reviewed by: alc, markj
Differential Revision: https://reviews.freebsd.org/D49015