Don't deprecate 'exclude'

Need to track outcome of Django's #19733, and decide on approach
accordingly.

rest_framework/serializers.py

@@ -165,11 +165,6 @@ class BaseSerializer(Field):
 
         # Remove anything in 'exclude'
         if self.opts.exclude:
-            # Note: To be deprecated in line with Django's ModelForm change.
-            # https://code.djangoproject.com/ticket/19733
-            warnings.warn('`exclude` option on serializers is due to be deprecated. '
-                          'Use the `fields` option instead.',
-                           PendingDeprecationWarning, stacklevel=2)
             for key in self.opts.exclude:
                 ret.pop(key, None)