EnumsGen: remove dangling assertion

StringAttr-backed enum attribute cases changed to allow explicit values,
But this assertion was not deleted.

Fixes https://github.com/tensorflow/mlir/issues/39

PiperOrigin-RevId: 256090793

mlir/lib/TableGen/Attribute.cpp

@@ -149,7 +149,6 @@ StringRef tblgen::EnumAttrCase::getSymbol() const {
 }
 
 int64_t tblgen::EnumAttrCase::getValue() const {
-  assert(isStrCase() && "cannot get value for EnumAttrCase");
   return def->getValueAsInt("value");
 }