Experiment: Simplify split modulestore
Doing some experiments.
This simply removes some functions from split modulestore that appear to be unused. I'm mostly pushing to get a CI build and see what happens.
Ensure nothing is broken with Studio's XBlock/course CRUD workflow and viewing XBlocks in the LMS.
This is just an experiment, not meant to be merged for now.
I also noticed quite a bit of code related to looking up a course based on its `wiki_slug`, which is only used for one specific thing: to check the staff permissions of course wiki pages. If there were some other way to look up courses by `wiki_slug`, then there's a whole bunch more code that would be unused.