[RISCV] Remove Zbproposedc extension

This consists of 3 compressed instructions, c.not, c.neg, and c.zext.w.
I believe these have been picked up by the Zce effort using different
encodings. I don't think it makes sense to keep them in bitmanip. It
will eventually cause a conflict if/when Zce is implemented in llvm.

Differential Revision: https://reviews.llvm.org/D110871