feat!: remove library_sourced block (#33257)

Originally, we planned to add support for V2 libraries and for static
(hand-selected) library block reference via new block type: library_sourced.

We have since decided that it would be better to add those capabilities
in-place to the existing library_content block. This will ease V1->V2
library migration and make adoption of the new features easier for current
library users. It will also avoid duplication of logic between two block types,
we we fear would be error-prone. For details, see this ADR:
 https://github.com/openedx/edx-platform/pull/33231

So, we are removing the library_sourced block.
This block has existed in edx-platform for a few years now, but was not
enabled by default and never officially supported. It was only usable via the
experimental V2 content library feature. Operators who added library_sourced
blocks to their course will now see them render as `HiddenBlock` instances, i.e.:

> ERROR: "library_sourced" is an unknown component type...

This should not impact other component types in such courses and should not
impact import/export.

openedx/core/lib/xblock_utils/__init__.py

@@ -452,7 +452,7 @@ def xblock_resource_pkg(block):
        ProblemBlock, and most other built-in blocks currently. Handling for these
        assets does not interact with this function.
     2. The (preferred) standard XBlock runtime resource loading system, used by
-       LibrarySourcedBlock. Handling for these assets *does* interact with this
+       LibraryContentBlock. Handling for these assets *does* interact with this
        function.
 
     We hope to migrate to (2) eventually, tracked by: