Opportunistically Remove Old Mongo Tests

[ormsbee]

We have many content-related tests that run either explicitly for the Old Mongo Modulestore (ModuleStoreEnum.Type.mongo), or use ddt to run under both Old and Split Modulestores. We can remove the Old Mongo-specific portions of many of these tests, with a few caveats:

• No tests related to CourseOverview compilation should be removed.
• No tests related to writing or reading of the root CourseBlock should be removed.

Do a first pass at searching the tests that exercise both Old and Split Mongo explicitly, and remove the Old Mongo tests. Measure the overall difference in test run-time before and after this work is done.