[RISCV] Remove unused variable 'ST' in RISCVTargetMachine.cpp (NFC)

llvm-project/llvm/lib/Target/RISCV/RISCVTargetMachine.cpp:358:27:
error: unused variable 'ST' [-Werror,-Wunused-variable]
    const RISCVSubtarget &ST = C->MF->getSubtarget<RISCVSubtarget>();
                          ^
1 error generated.

Author: DamonFool

llvm/lib/Target/RISCV/RISCVTargetMachine.cpp

@@ -355,7 +355,6 @@ class RISCVPassConfig : public TargetPassConfig {
 
   ScheduleDAGInstrs *
   createMachineScheduler(MachineSchedContext *C) const override {
-    const RISCVSubtarget &ST = C->MF->getSubtarget<RISCVSubtarget>();
     ScheduleDAGMILive *DAG = nullptr;
     if (EnableMISchedLoadClustering) {
       DAG = createGenericSchedLive(C);