Syncing with core: Drop the goals and tags fields from Store

Apparently no migration is needed since these are not actual fields,
but managers.
Syncing with core: Drop the goals and tags fields from TranslationProject

Apparently no migration is needed since these are not actual fields,
but managers.
Syncing with core: Drop goals and tags

All the goals and tags code included in the other apps is removed. Only
remnants of pootle_tagging app are kept in order to allow running the
migrations.