Stop using `alternative_title` field

The `alternative_title` field has been removed from use from the
publisher application as of alphagov/publisher#292.

With this change, we stop using the field in frontend (where it's
exposed to the user) so that it can later be removed from the Content
API.

Note: the majority of the files changed here are test fixtures that
contained this field

Story: https://www.agileplannerapp.com/boards/173808/cards/8145