[llvm] [RISCV] Removed unused OPERAND_SIMM8. NFC (PR #166215)

[Craig Topper via llvm-commits]

topperc wrote:

> > This was presumably replaced with the SIMM8_UNSIGNED? If so LGTM.
> 
> That's my guess, but I didn't check.

Confirmed.

https://github.com/llvm/llvm-project/pull/166215