riscv: add an assert to pmap_remove_pages()

Similar checks exist for both arm64 and amd64, but note that for amd64
it is a bare panic().

Reviewed by: markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36564

(cherry picked from commit 99fe523778af8cd0db3a17b253adb77a980176be)