[mlir] Fix a warning

This patch fixes:

  llvm-project/mlir/lib/Dialect/Linalg/TransformOps/LinalgTransformOps.cpp:1715:28:
  error: unused variable 'kPadToMultipleOfKeyword'
  [-Werror,-Wunused-const-variable]

Author: kazutakahirata

mlir/lib/Dialect/Linalg/TransformOps/LinalgTransformOps.cpp

@@ -1712,8 +1712,6 @@ transform::PackTransposeOp::apply(transform::TransformRewriter &rewriter,
 // PadOp
 //===---------------------------------------------------------------------===//
 
-static const StringLiteral kPadToMultipleOfKeyword = "pad_to_multiple_of";
-
 void transform::PadOp::build(OpBuilder &b, OperationState &result, Value target,
                              ArrayRef<int64_t> paddingDimensions,
                              ArrayRef<int64_t> padToMultipleOf,