[RISCV] Remove getPostRAMutations (#117527)

We are using `PostMachineScheduler` instead of `PostRAScheduler`
since #68696.

The hook `getPostRAMutations` is only used in `PostRAScheduler` so
it is actually dead code for RISC-V now.

Author: wangpc-pp

llvm/lib/Target/RISCV/RISCVSubtarget.cpp

@@ -186,11 +186,6 @@ bool RISCVSubtarget::useRVVForFixedLengthVectors() const {
 
 bool RISCVSubtarget::enableSubRegLiveness() const { return true; }
 
-void RISCVSubtarget::getPostRAMutations(
-    std::vector<std::unique_ptr<ScheduleDAGMutation>> &Mutations) const {
-  Mutations.push_back(createMacroFusionDAGMutation(getMacroFusions()));
-}
-
   /// Enable use of alias analysis during code generation (during MI
   /// scheduling, DAGCombine, etc.).
 bool RISCVSubtarget::useAA() const { return UseAA; }

llvm/lib/Target/RISCV/RISCVSubtarget.h

@@ -301,9 +301,6 @@ class RISCVSubtarget : public RISCVGenSubtargetInfo {
 
   bool enableSubRegLiveness() const override;
 
-  void getPostRAMutations(std::vector<std::unique_ptr<ScheduleDAGMutation>>
-                              &Mutations) const override;
-
   bool useAA() const override;
 
   unsigned getCacheLineSize() const override {