[GlobalISel] Remove unused variable 'ResultTy' in CombinerHelper.cpp (NFC)

llvm-project/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp:1179:7:
 error: unused variable 'ResultTy' [-Werror,-Wunused-variable]
  LLT ResultTy = MRI.getType(MI.getOperand(0).getReg());
      ^
1 error generated.