Simplified if statement to one line solution

Based on feedback on the pull request, changed solution to
be simpler for issue 3042

rest_framework/relations.py

@@ -42,8 +42,7 @@ class RelatedField(Field):
             'Relational fields should not provide a `queryset` argument, '
             'when setting read_only=`True`.'
         )
-        if 'many' in kwargs:
-            kwargs.pop('many')
+        kwargs.pop('many', None)
         super(RelatedField, self).__init__(**kwargs)
 
     def __new__(cls, *args, **kwargs):