arm64/vmm: Simplify vmmpmap_remove()

This function handles invalid intermediate PTP entries, but there's no
reason for this: the passed range should be mapped. In particular, we
assert that all L3 entries encountered are mapped. So let's simplify
the code a bit. No functional change intended.

Reviewed by: andrew
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D54738