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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[Yagnesh1998]

I will create a new pull request with changes.

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