[CodeGen] Remove unused variables in TargetLoweringBase.cpp (NFC)

llvm-project/llvm/lib/CodeGen/TargetLoweringBase.cpp:570:12: error: unused variable 'ModeN' [-Werror,-Wunused-variable]
  570 |   unsigned ModeN, ModelN;
      |            ^~~~~
llvm-project/llvm/lib/CodeGen/TargetLoweringBase.cpp:570:19: error: unused variable 'ModelN' [-Werror,-Wunused-variable]
  570 |   unsigned ModeN, ModelN;
      |                   ^~~~~~
2 errors generated.