frag6: Reduce code duplication

The code which removes a fragment queue from the per-VNET hash table was
duplicated three times. Factor it out into a function. No functional
change intended.

Reviewed by: kp, bz
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43228

(cherry picked from commit 0736a38072b52204289c669770a34d0b801a8a7e)