refactor: Completely remove Blockstore (#34739)

Blockstore and all of its (experimental) functionality has been replaced with
openedx-learning, aka "Learning Core". This commit uninstalls the now-unused
openedx-blockstore package and removes all dangling references to it.

Note: This also removes the `copy_library_from_v1_to_v2` management command,
which has been broken ever since we switched from Blockstore to Learning Core.

Part of this DEPR: https://github.com/openedx/public-engineering/issues/238

xmodule/tests/test_library_tools.py

@@ -28,7 +28,7 @@ class ContentLibraryToolsTest(MixedSplitTestCase, ContentLibrariesRestApiTest):
     """
     Tests for LibraryToolsService.
 
-    Tests interaction with blockstore-based (V2) and mongo-based (V1) content libraries.
+    Tests interaction with learning-core-based (V2) and mongo-based (V1) content libraries.
     """
     def setUp(self):
         super().setUp()