cmd/compile: remove special handling for OpRISCV64SUBW in markUnsafePoints

Due to improved optimisation, we no longer emit SUBW for the write barrier
checks on riscv64, hence remove special handling in markUnsafePoints.

Change-Id: Ia1150c3e11f25e183735e58f8716a511d9e90fb3
Reviewed-on: https://go-review.googlesource.com/c/go/+/222638


Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>