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/html_block.py

@@ -279,7 +279,7 @@ class HtmlBlockMixin(  # lint-amnesty, pylint: disable=abstract-method
     @classmethod
     def parse_xml_new_runtime(cls, node, runtime, keys):
         """
-        Parse XML in the new blockstore-based runtime. Since it doesn't yet
+        Parse XML in the new learning-core-based runtime. Since it doesn't yet
         support loading separate .html files, the HTML data is assumed to be in
         a CDATA child or otherwise just inline in the OLX.
         """