[RISCV] Fix a warning

This patch fixes:

  llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp:476:25: error: unused
  variable 'ST' [-Werror,-Wunused-variable]

Author: kazutakahirata

llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp

@@ -473,7 +473,6 @@ bool RISCVRegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator II,
   MachineInstr &MI = *II;
   MachineFunction &MF = *MI.getParent()->getParent();
   MachineRegisterInfo &MRI = MF.getRegInfo();
-  const RISCVSubtarget &ST = MF.getSubtarget<RISCVSubtarget>();
   DebugLoc DL = MI.getDebugLoc();
 
   int FrameIndex = MI.getOperand(FIOperandNum).getIndex();