riscv: Remove the unused riscv64_cpu driver

This is a repeat of 63bf2d735ca3 ("Remove the unused arm64_cpu driver.")
for RISC-V, which copied the defunct code from arm64 with no changes
beyond substituting riscv64 for arm64, and made no use of it elsewhere.
It has thus always been entirely superfluous.

Reviewed by: mhorne
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43672