Remove AnySimpleType.xmlvalue() method override

Since the base class (AnyType) now implements serialization guessing we
can use that here too.

Author: mvantellingen

src/zeep/xsd/types/simple.py

@@ -79,7 +79,3 @@ def signature(self, schema=None, standalone=True):
     def validate(self, value, required=False):
         if required and value is None:
             raise ValidationError("Value is required")
-
-    def xmlvalue(self, value):
-        raise NotImplementedError(
-            '%s.xmlvalue() not implemented' % self.__class__.__name__)