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