feat!: remove blockstore support

Blockstore was completely removed from edx-platform in:
  openedx/edx-platform#34739

As a follow-on to that work, this commit removes the entirety of two
blockstore-related plugins (tutor-contrib-blockstore-filesystem and
tutor-contrib-blockstore-minio), as well as the blockstore configuration
present in tutor-contrib-library-authoring-mfe.

README.rst

@@ -20,8 +20,6 @@ Plugin                               Status (*Experimental*, *Production*, or *D
 ===================================  ======================================================
 tutor-contrib-learner-dashboard-mfe  Deprecated
 tutor-contrib-library-authoring-mfe  Experimental
-tutor-contrib-blockstore-filesystem  Experimental
-tutor-contrib-blockstore-minio       Experimental
 ===================================  ======================================================
 
 Getting Started
@@ -31,7 +29,7 @@ Generally, assuming `Tutor is installed`_, to add a plugin in this repository
 to your deployment environment you would:
 
 .. code:: bash
-	  
+
 	  git clone [email protected]:openedx/openedx-tutor-plugins
           cd openedx-tutor-plugins/plugins/<plugin-name>
           pip install -e .