Breaking change in 26 release: Remove msg.UnknownFields() support in pure python and cpp extension.

Users should use the add-on unknown_fields.py support.
Old usage example:
unknown_field_set = msg.UnknownFields()

New usage should be:
from google.protobuf import unknown_fields
unknown_field_set = unknown_fields.UnknownFieldSet(msg)
PiperOrigin-RevId: 589969095

python/google/protobuf/internal/python_message.py

@@ -1385,15 +1385,9 @@ def _Clear(self):
 
 
 def _UnknownFields(self):
-  warnings.warn(
-      'message.UnknownFields() is deprecated. Please use the add one '
-      'feature unknown_fields.UnknownFieldSet(message) in '
-      'unknown_fields.py instead.'
-  )
-  if self._unknown_field_set is None:  # pylint: disable=protected-access
-    # pylint: disable=protected-access
-    self._unknown_field_set = containers.UnknownFieldSet()
-  return self._unknown_field_set    # pylint: disable=protected-access
+  raise NotImplementedError('Please use the add-on feaure '
+                            'unknown_fields.UnknownFieldSet(message) in '
+                            'unknown_fields.py instead.')
 
 
 def _DiscardUnknownFields(self):

python/google/protobuf/pyext/message.cc

@@ -2346,16 +2346,11 @@ static PyObject* GetExtensionDict(CMessage* self, void *closure) {
 }
 
 static PyObject* GetUnknownFields(CMessage* self) {
-  PyErr_Warn(nullptr,
-             "message.UnknownFields() is deprecated. Please use the "
-             "add one feature unknown_fields.UnknownFieldSet(message) in "
-             "unknown_fields.py instead.");
-  if (self->unknown_field_set == nullptr) {
-    self->unknown_field_set = unknown_fields::NewPyUnknownFields(self);
-  } else {
-    Py_INCREF(self->unknown_field_set);
-  }
-  return self->unknown_field_set;
+  PyErr_Format(PyExc_NotImplementedError,
+               "Please use the add-on feature "
+               "unknown_fields.UnknownFieldSet(message) in "
+               "unknown_fields.py instead.");
+  return nullptr;
 }
 
 static PyGetSetDef Getters[] = {