feat: block_structure.storage_backing_for_cache toggle removed depr32

[Yagnesh1998]

Description
As Per this DEPR: openedx/public-engineering#32

Removing: Toggle labeled 'block_structure.storage_backing_for_cache'. The toggle has the following annotation as a description:

When enabled, block structures are stored in a more permanent storage, like a database, which provides an additional backup for cache misses, instead having them regenerated. The regeneration of block structures is a time consuming process. Therefore, enabling this switch is recommended for Production.

Supporting information
as per the original jeera issue https://openedx.atlassian.net/browse/DEPR-145, this toggle is removed

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[mphilbrick211]

Hi @Yagnesh1998! Just flagging that there are some failing tests here.

[Yagnesh1998]

@mphilbrick211 I am working on it. thanks

[mphilbrick211]

Hi @Yagnesh1998! Just flagging that there are some failing tests here.

Friendly ping on this @Yagnesh1998!

[Yagnesh1998]

@dianakhuang all test are passed successfully let me know if we need to do some changes.
Thanks

[dianakhuang]

I think I just had one nitpick, but other than that, I think this looks good.

[dianakhuang]

nit: this no longer returns a StubModel, so we should update this docstring.

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[Yagnesh1998]

We close this PR and create another PR according to the multiple commits in this PR.