Removal of the callback get_shortcuts() and its plugin implementations.
The private methods in the content_item_readonly_repository class
which were only used by the callback hook logic are also removed.
This involves a few changes:
- The callback is still called by the repository but only if the
replacement callback get_course_content_items is not implemented.
- Debugging is called when the get_shortcuts callback is called.
- We leave lti's implementation of get_shortcuts alone, in order to
maintain the output of get_module_metadata during its deprecation life
span.
