arm64: Remove a racy KASSERT from pmap_remove_pages()

PCPU_GET(curpmap) expands to multiple instructions on arm64, and if the
current thread is migrated in between execution of those instructions, a
stale value may be used in the assertion condition.

Diagnosed by: mmel
Reported by: mmel, Bob Prohaska <fbsd@www.zefox.net>
Submitted by: alc
MFC after: 1 week