[AArch64] Fix -Wunused-variable in AArch64FrameLowering.cpp (NFC)

llvm-project/llvm/lib/Target/AArch64/AArch64FrameLowering.cpp:3084:31:
error: unused variable 'Subtarget' [-Werror,-Wunused-variable]
      const AArch64Subtarget &Subtarget = MF.getSubtarget<AArch64Subtarget>();
                              ^
llvm-project/llvm/lib/Target/AArch64/AArch64FrameLowering.cpp:3253:31:
error: unused variable 'Subtarget' [-Werror,-Wunused-variable]
      const AArch64Subtarget &Subtarget = MF.getSubtarget<AArch64Subtarget>();
                              ^
2 errors generated.