vmm: Remove an unneeded NULL pointer check

sc->vm is unconditionally dereferenced earlier in this function. No
functional change intended.

Reviewed by: bnovkov
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D55069

(cherry picked from commit 2d0564b9daf49c46e0e4ef6c7d162c73fc8b35d7)