riscv: Retire PMAP_INLINE

pmap_kremove() is not called from within pmap.c, so there's no reason to
inline it. No functional change intended.

Reviewed by: alc, kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42287

(cherry picked from commit 953345922398ae6c029713087c8494a346d90c85)