[RISCV] Remove SRO* and SLO* instructions from bitmanip.

As of the current draft these are no longer being considered
for the bitmanip spec. It wasn't clear what sub extension they
belonged in in the 0.93 spec.

So remove them. They can always be added back if something changes.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D96157

(cherry picked from commit fd5adae0)