[RISCV] Remove XSfcie extension.

This reverts 0d3eee33 and
4c37d30e.

XSfcie is not an official SiFive extension name. It stands for
SiFive Custom Instruction Extension, which is mentioned in the S76
manual, but then elsewhere in the manual says it is not supported
for S76.

LLVM had various instructions and CSRs listed as part of this
extension, but as far as SiFive is concerned, none of them are part
of it. There are no documented extension names for these instructions
and CSRs either externally or internally.

If these are important to LLVM users, I can facilitate creating
extension names for them and have them documented. For now I'm
removing everything.

Unfortunately, these instructions and CSRs are in LLVM 17 so this
is an incompatible change.