Fix overzealous RelatedField queryset validation.

Otherwise if you want to subclass RelatedField in a read+write capacity you end up with this weird situation:
```
    class MyField(serializers.RelatedField):
        def __init__(self, **kwargs):
            if not kwargs.get('read_only'):
                kwargs.setdefault('queryset', MyModel.objects.all())
            super(MyField, self).__init__(**kwargs)
```

This commit removes the check preventing you from just supplying the queryset anyway. Why shouldn't you?

rest_framework/relations.py

@@ -69,10 +69,6 @@ class RelatedField(Field):
             'Relational field must provide a `queryset` argument, '
             'or set read_only=`True`.'
         )
-        assert not (self.queryset is not None and kwargs.get('read_only', None)), (
-            'Relational fields should not provide a `queryset` argument, '
-            'when setting read_only=`True`.'
-        )
         kwargs.pop('many', None)
         kwargs.pop('allow_empty', None)
         super(RelatedField, self).__init__(**kwargs)