Removed backwards compatibility code to call field.widget._has_changed()

This logic should be moved to field._has_changed() as described
in ebb504db69 - refs #16612.

django/forms/forms.py

@@ -12,7 +12,7 @@ from django.core.exceptions import ValidationError, NON_FIELD_ERRORS
 from django.forms.fields import Field, FileField
 from django.forms.utils import flatatt, ErrorDict, ErrorList
 from django.forms.widgets import Media, MediaDefiningClass, TextInput, Textarea
-from django.utils.deprecation import RemovedInDjango18Warning, RemovedInDjango19Warning
+from django.utils.deprecation import RemovedInDjango19Warning
 from django.utils.encoding import smart_text, force_text, python_2_unicode_compatible
 from django.utils.html import conditional_escape, format_html
 from django.utils.safestring import mark_safe
@@ -429,13 +429,7 @@ class BaseForm(object):
                         # Always assume data has changed if validation fails.
                         self._changed_data.append(name)
                         continue
-                if hasattr(field.widget, '_has_changed'):
-                    warnings.warn("The _has_changed method on widgets is deprecated,"
-                        " define it at field level instead.",
-                        RemovedInDjango18Warning, stacklevel=2)
-                    if field.widget._has_changed(initial_value, data_value):
-                        self._changed_data.append(name)
-                elif field._has_changed(initial_value, data_value):
+                if field._has_changed(initial_value, data_value):
                     self._changed_data.append(name)
         return self._changed_data