feat!: Drop the block_structure.storage_backing_for_cache waffle switch.

This work is part of DEPR openedx/public-engineering#32

Now that we've removed all uses for this switch remove the decleration
as well.

BREAKING CHANGE: The `block_structure.storage_backing_for_cache` will no
longer exist and its value will be ignored. If you have this switch set
in your instance you can remove it. The backing cache is now always ON.

openedx/core/djangoapps/content/block_structure/config/__init__.py

@@ -9,28 +9,6 @@
 
 from .models import BlockStructureConfiguration
 
-# Switches
-# .. toggle_name: block_structure.storage_backing_for_cache
-# .. toggle_implementation: WaffleSwitch
-# .. toggle_default: False
-# .. toggle_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 regenration of block structures is a time consuming process. Therefore,
-#   enabling this switch is recommended for Production.
-# .. toggle_warning: Depends on `BLOCK_STRUCTURES_SETTINGS['STORAGE_CLASS']` and
-#   `BLOCK_STRUCTURES_SETTINGS['STORAGE_KWARGS']`.
-#   This switch will likely be deprecated and removed.
-#   The annotation will be updated with the DEPR ticket once that process has started.
-# .. toggle_use_cases: temporary
-# .. toggle_creation_date: 2017-02-23
-# .. toggle_target_removal_date: 2017-05-23
-# .. toggle_tickets: https://github.com/openedx/edx-platform/pull/14512,
-#   https://github.com/openedx/edx-platform/pull/14770,
-#   https://openedx.atlassian.net/browse/DEPR-145
-STORAGE_BACKING_FOR_CACHE = WaffleSwitch(
-    "block_structure.storage_backing_for_cache", __name__
-)
-
 
 @request_cached()
 def num_versions_to_keep():