Deprecate the *_get_completion_state() callbacks

With the activity_custom_completion base class being implemented by activity modules supporting custom completion rules, it is time to deprecate these callbacks as they won't be used anymore.

A possible approach to do the deprecation:

1. Add a debugging message that these callbacks are now being deprecated in completion_info::internal_get_state()
2. Move these callbacks to the plugin's deprecatedlib.php. E.g. in assign, move to mod/assign/deprecatedlib.php. Make sure that deprecatedlib.php is included in the plugin's lib.php
3. Add the related PHPDoc block tags (e.g. @deprecated, @todo, @see) to the *_get_completion_state() callbacks.
4. Note the deprecation of these callbacks in upgrade.txt