fix: Remove pointless Maintenance and Announcement apps

The Studio Maintenance app had two features:

* "Force Course Publish", which literally doesn't do anything. All it
  does is tell you what version *would* be seen by users *if* the course
  were to be published--no publishing actually occurs via this feature.

* "Announcements", which writes to the announcements_announcement
  database table, but doesn't actually display anywhere.

Having these pages in the platform is actively misleading and creates a
maintenance burden for edx-platform developers, so we remove them.

Note that this commit does not include a migration for the announcements
Django app. So, announcements_announcement table will not be deleted.
Given the small expected size of any past-authored announcements, we are
not worried about leaving them in the database perpetually.