[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.

Author: DamonFool

mlir/lib/Dialect/Vector/Transforms/VectorEmulateNarrowType.cpp

@@ -760,7 +760,6 @@ static LogicalResult alignedConversionPrecondition(PatternRewriter &rewriter,
     return rewriter.notifyMatchFailure(op, "Not a supported aligned case");
   unsigned srcElemBitwidth = srcType.getElementTypeBitWidth();
   unsigned dstElemBitwidth = dstType.getElementTypeBitWidth();
-  unsigned byteBitwidth = 8;
 
   // Only {s}i4 -> (size_of({{s}i/f}) >= 8) are supported for now.
   if (srcElemBitwidth != 4 || dstElemBitwidth < 8 ||