[RISCV] Fix a warning

This patch fixes:

  llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp:579:11: error: unused
  variable 'Subtarget' [-Werror,-Wunused-variable]

Author: kazutakahirata

llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp

@@ -576,7 +576,6 @@ bool RISCVRegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator II,
     int64_t Val = Offset.getFixed();
     int64_t Lo12 = SignExtend64<12>(Val);
     unsigned Opc = MI.getOpcode();
-    auto &Subtarget = MF.getSubtarget<RISCVSubtarget>();
 
     if (Opc == RISCV::ADDI && !isInt<12>(Val)) {
       // We chose to emit the canonical immediate sequence rather than folding