riscv: remove a redundant check on PGA_WRITEABLE

This is achieved by the preceding call to pmap_page_is_write_mapped().
It appears the second check and comment were left mistakenly when the
intent was to remove them.

Reviewed by: mhorne
MFC after: 1 week
Fixes: 638f867814a6 ("(6/6) Convert pmap to expect busy in write... ")
Pull Request: https://github.com/freebsd/freebsd-src/pull/1304