[RISCV] Remove unused variables in RISCVISelDAGToDAG.cpp (NFC)

[RISCV] Remove unused variables in RISCVISelDAGToDAG.cpp (NFC)

/Users/jiefu/llvm-project/llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp:97:33: error: unused variable 'FuncInfo' [-Werror,-Wunused-variable]
      RISCVMachineFunctionInfo *FuncInfo =
                                ^
/Users/jiefu/llvm-project/llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp:106:29: error: unused variable 'TLI' [-Werror,-Wunused-variable]
      const TargetLowering &TLI = CurDAG->getTargetLoweringInfo();
                            ^
2 errors generated