feat!: assume & remove BLOCKSTORE_USE_BLOCKSTORE_APP_API

[kdmccormick]

Originally, Blockstore was an independent micro-service, accessed via a REST API.
Then, we changed Blockstore so it could be installed as an in-process Django app.

To support both modes, there existed a blockstore_api wrapper library in edx-platform,
with toggles controlling whether the wrapper called out to the micro-service's REST API versus the
Django app's Python API. Now that the micro-service Blockstore implementation is deprecated,
though, this wrapper library and toggles are just unnecessary complexity.

As a first step towards cleanup, we:

• remove several toggles and settings (details below);
• remove the blocokstore_api wrapper methods which called the REST API and
  marshalled them back into Python objects; and
• remove all test cases which relied on the Blockstore micro-service (and were skippped in CI).

In the future, we will remove the content libraries indexer,
clean up the remaining bits of blockstore_api, and flatten out all
the Blockstore-related test class hierarchies which are no longer ncessary.

BREAKING CHANGE:

• These Django settings are removed:
  • BLOCKSTORE_PUBLIC_URL_ROOT
  • BLOCKSTORE_API_URL
  • BLOCKSTORE_API_AUTH_TOKEN
  • BLOCKSTORE_USE_BLOCKSTORE_APP_API
• The blockstore.use_blockstore_app_api Waffle switch is removed.
• edx-platform will act as it did when the DJango setting BLOCKSTORE_USE_BLOCKSTORE_APP_API
  or the Waffle switch blockstore.use_blockstore_app_api were enabled. That is, any running Blockstore
  micro-service instance will be ignored, and the Blockstore package which is installed into edx-platform
  will be used instead.

Ref: openedx-unsupported/blockstore#296

[connorhaugh]

Super fast pass, but looks normal to me.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX staging environment in preparation for a release to production.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.