[mlir] Remove unused variable 'byteBitwidth' in VectorEmulateNarrowType.cpp (NFC)

llvm-project/mlir/lib/Dialect/Vector/Transforms/VectorEmulateNarrowType.cpp:763:12:
error: unused variable 'byteBitwidth' [-Werror,-Wunused-variable]
  unsigned byteBitwidth = 8;
           ^
1 error generated.