[mlir][arith] Delete unnecessary error logs

Function `getNeutralElement` already indicates "cannot find neutral element"
by returning nullptr through the return value, and no additional error log needs
to be output.

Author: cxy-1993

mlir/lib/Dialect/Arith/IR/ArithOps.cpp

@@ -2544,7 +2544,6 @@ std::optional<TypedAttr> mlir::arith::getNeutralElement(Operation *op) {
           .Case([](arith::MulIOp op) { return AtomicRMWKind::muli; })
           .Default([](Operation *op) { return std::nullopt; });
   if (!maybeKind) {
-    op->emitError() << "Unknown neutral element for: " << *op;
     return std::nullopt;
   }