fix: Drop mock STATICFILES_STORAGE_KWARGS

[robrap]

The setting was misleading, because it doesn't
do anything as-is. The code that reads from this
dict uses the default of
settings.STORAGES['staticfiles']['BACKEND'], which is
openedx.core.storage.ProductionStorage, as the key.
The data under the defined (and now removed) key
openedx.core.storage.ProductionS3Storage
would never have been used.

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[timmc-edx]

"A different key", meaning in place of openedx.core.storage.ProductionS3Storage, right?

[robrap]

"A different key", meaning in place of openedx.core.storage.ProductionS3Storage, right?

@timmc-edx: Yes. It would use settings.STORAGES['staticfiles']['BACKEND'] default, which is openedx.core.storage.ProductionStorage.