[OldMongo FC-0004] Remove inheritance-related code from Old Mongo

[UvgenGen]

Description

• The inheritance-related code is safely removed from the codebase.
• Tests related to inheritance from old mongo are fixed

Useful information to include:
openedx/public-engineering#79

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[ormsbee]

@jristau1984: this will not merge until Monday at the earliest (2022-10-17).

[ormsbee]

Code changes look good. Minor points:

1. Can we get rid of some of this performance testing code rather than adjusting the numbers upward?
2. Removal of features should have a feat!: conventional commit syntax.

FYI @jristau1984: If there is an unexpected use of Old Mongo for course content, you may see increased MongoDB load in New Relic after this PR merges, because it removes part of Old Mongo's internal caching layer. I don't really expect this to happen, and if it does happen, I think that the relatively small number of Old Mongo courses means that it won't threaten site stability. But it might be a useful canary test before the next set of removals.

@jmbowman: There is a chance that this PR merge will temporarily make the test suite slower (until #31134 merges after it).

[ormsbee]

Are these numbers higher because this test is running against Old Mongo instead of Split? If so, can we convert it to split? And if it's purely testing Old Mongo performance (i.e. not correctness), can we get rid of it altogether rather than running it more slowly?

[ormsbee]

Same question as above about whether this can be converted to Split or removed altogether.

[UvgenGen]

Yes, this changes will be converted to Split in #31134. I leaved this test with Old Mongo modulestore for checking. Should I convert to split in this PR?

[UvgenGen]

I rechecked #31134 , and this test are broken when I use split modulestore. So I just added skip for this tests in that PR(#31134). I think we really can get rid of this tests, can't we?.

[ormsbee]

Yes, we can get rid of them.

• If there are tests that never worked for Split Mongo, we should get rid of them.
• If there are tests that specifically test Old Mongo performance, we should probably get rid of them.
• If there are tests that exercise Old Mongo functionality that we intend to remove, we can get rid of them now, or save them for another PR where removing them is more natural.

[UvgenGen]

The tests have been updated, the rest of the tests will be in the next PR

[UvgenGen]

@ormsbee commit message updated with feat!:, PR rebased

[ormsbee]

@jristau1984: As a heads up, I plan to merge this Old Mongo PR on Monday morning.

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EdX Release Notice: This PR has been deployed to the staging environment in preparation for a release to production.

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EdX Release Notice: This PR has been deployed to the production environment.