This commit leaves behind just enough Old Mongo (DraftModulestore)
functionality to allow read-only access to static assets and the
root CourseBlock. It removes:
* create/update operations
* child/parent traversal
* inheritance related code
It also removes or converts tests for this functionality.
The ability to read from the root CourseBlock was maintained for
backwards compatibility, since top-level course settings are often
stored here, and this is used by various parts of the codebase,
like displaying dashboards and re-building CourseOverview models.
Any attempt to read the contents of a course by getting the
CourseBlock's children will return an empty list (i.e. it will look
empty).
This commit does _not_ delete content on MongoDB or run any sort of
data migration or cleanup.
c5d1807c81