Remove code to support BBB in the app as a plugin

In 4.0 we decided to keep the code to support BBB in the Moodle app as an external plugin because users could take a while to update the app to 4.0. If we had removed that code, the BBB activity wouldn't have worked in previous versions of the app.

Now most users should have updated the app already, so IMO this code can be removed for the 4.1 version. Basically the files that need to be removed are:

• db/mobile.php
• classes/output/mobile.php
• templates/mobile_*

This code is specific for the app, so I guess it can be removed without following the deprecation policy but it would be nice if an integrator can confirm it.