[RISCV][GlobalISel] Remove unused variable 'Index' (NFC)

/data/llvm-project/llvm/lib/Target/RISCV/GISel/RISCVCallLowering.cpp:272:12: error: variable 'Index' set but not used [-Werror,-Wunused-but-set-variable]
  unsigned Index = 0;
           ^
1 error generated.