[RISCV] Remove CSR names that are not in Privileged Specification 1.12.

These are from the N extension (User-Level Interrupts) which did
not make it into 1.12 of the Privileged Specification.

D117653 also tried to remove some of these, but it was never reviewed.

Reviewed By: jrtc27

Differential Revision: https://reviews.llvm.org/D149278