feat!: Remove outdated Libraries Relaunch cruft (#35644)

The V2 libraries project had a few past iterations which were never
launched. This commit cleans up pieces from those which we don't need
for the real Libraries Relaunch MVP in Sumac:

* Remove ENABLE_LIBRARY_AUTHORING_MICROFRONTEND,
  LIBRARY_AUTHORING_FRONTEND_URL, and
  REDIRECT_TO_LIBRARY_AUTHORING_MICROFRONTEND, all of which are obsolete
  now that library authoring has been merged into
  https://github.com/openedx/frontend-app-authoring.
  More details on the new Content Libraries configuration settings are
  here: https://github.com/openedx/frontend-app-authoring/issues/1334

* Remove dangling support for syncing V2 (learning core-backed) library
  content using the LibraryContentBlock. This code was all based on an
  older understanding of V2 Content Libraries, where the libraries were
  smaller and versioned as a whole rather then versioned by-item.
  Reference to V2 libraries will be done on a per-block basis using
  the upstream/downstream system, described here:
  https://github.com/openedx/edx-platform/blob/master/docs/decisions/0020-upstream-downstream.rst
  It's important that we remove this support now so that OLX course
  authors don't stuble upon it and use it, which would be buggy and
  complicate future migrations.

* Remove the "mode" parameter from LibraryContentBlock. The only
  supported mode was and is "random". We will not be adding any further
  modes. Going forward for V2, we will have an ItemBank block for
  randomizing items (regardless of source), which can be synthesized
  with upstream referenced as described above. Existing
  LibraryContentBlocks will be migrated.

* Finally, some renamings:

  * LibraryContentBlock -> LegacyLibraryContentBlock
  * LibraryToolsService -> LegacyLibraryToolsService
  * LibrarySummary -> LegacyLibrarySummary

  Module names and the old OLX tag (library_content) are unchanged.

Closes: https://github.com/openedx/frontend-app-authoring/issues/1115

webpack.builtinblocks.config.js

@@ -38,6 +38,10 @@ module.exports = {
             './xmodule/js/src/xmodule.js',
             './xmodule/js/src/html/edit.js'
         ],
+        LibraryContentBlockEditor: [
+            './xmodule/js/src/xmodule.js',
+            './xmodule/js/src/vertical/edit.js'
+        ],
         LTIBlockDisplay: [
             './xmodule/js/src/xmodule.js',
             './xmodule/js/src/lti/lti.js'
@@ -46,11 +50,6 @@ module.exports = {
             './xmodule/js/src/xmodule.js',
             './xmodule/js/src/raw/edit/metadata-only.js'
         ],
-        LibraryContentBlockDisplay: './xmodule/js/src/xmodule.js',
-        LibraryContentBlockEditor: [
-            './xmodule/js/src/xmodule.js',
-            './xmodule/js/src/vertical/edit.js'
-        ],
         PollBlockDisplay: [
             './xmodule/js/src/xmodule.js',
             './xmodule/js/src/javascript_loader.js',