[RISCV] Remove FCSR from RISCVRegisterInfo.

We only used this to mark it as a reserved register. But that's not
important if we don't do anything else with it.

I think if we were ever to do anything with it, we would need to
model it as a super register of FRM and FFLAGS. But it might be
easier to reference both FRM and FFLAGS in implicit defs/uses
for anything we were to do with "fcsr".

Reviewed By: sepavloff

Differential Revision: https://reviews.llvm.org/D115455