[llvm] [RISCV] Remove getPostRAMutations (PR #117527)

[Craig Topper via llvm-commits]

topperc wrote:

> LGTM.
> 
> On another note, have we looked at using the MacroFusionDAGMutation for PostMachineSched?

It's handled in `llvm::createGenericSchedPostRA` already

https://github.com/llvm/llvm-project/pull/117527