Remove deprecated fields from models

Description

A few of our storage models have deprecated fields, these should be removed by a one-off job. You can look at #9329 where we removed username from all the commit log models.

List of deprecated fields

Models that inherit from BaseModel

• ExpSummaryModel → translator_ids @rohitkatlaa
• UserSettingsModel → gae_user_id @rohitkatlaa
• UserSubscriptionsModel → feedback_thread_ids @YashJipkate
• UserSubscriptionsModel → activity_ids @krishita30j

Nodels that inherit from VersionedModel

• CollectionModel → nodes @iramin (#11350)
• CollectionRightsModel → translator_ids @iramin (#11350)
• ExplorationModel → skill_tags @hardikkat24
• ExplorationModel → default_skin @hardikkat24
• ExplorationModel → skin_customizations @hardikkat24
• ExplorationRightsModel → translator_ids @hardikkat24
• ExplorationRightsModel → all_viewer_ids (it is not shown as deprecated but still contained in some older versions of the model) @hardikkat24

Steps to remove a field from models that inherit from BaseModel

1. Remove the field form the storage model code itself.
2. Add a migration job to remove the field from already existing models. The one-off job for removing the username from commit log models looks like this: https://github.com/oppia/oppia/blob/170bdeae5912ced0abc71257f5b0e5ca98fd1418/core/domain/activity_jobs_one_off.py#L142-L182
3. Create a PR and submit the job to be tested on the backup server.

Steps to remove a field from models that inherit from VersionedModel

1. Remove the field form the storage model code itself.
2. Add a migration job to remove the field from already existing models. The one-off job for removing the username from commit log models looks like this: https://github.com/oppia/oppia/blob/170bdeae5912ced0abc71257f5b0e5ca98fd1418/core/domain/activity_jobs_one_off.py#L142-L182
3. Add _reconstitute method for the model that will ensure that when we revert to older version of the model we properly remove the field so that the model can be loaded. _reconstitute is already implemented for the CollectionRightsModel and ExplorationRightsModel https://github.com/oppia/oppia/blob/7623cd028d15a6326cac186f673f368dcae30929/core/storage/exploration/gae_models.py#L403-L440
4. Create a PR and submit the job to be tested on the backup server.

[michaelw54]

Can we update the instructions for removing a field from VersionedModel inheriting classes to use model.commit() rather than model.put()? It seems that the put() method deliberately throws a NotImplementedError on VersionedModel as defined here:

def put(self, *args, **kwargs):
        """For VersionedModels, this method is replaced with commit()."""
        raise NotImplementedError(
            'The put() method is missing from the '
            'derived class. It should be implemented in the derived class.')

Just thought it may be helpful for new contributors working on similar tasks with VersionedModel

[DubeySandeep]

Assigning @sjiang647 to the ExplorationRightsModel model part of the issue. (As per offline discussion)