Most of the methods in `XmlMixin` act as wrappers for the official API for
serialization and deserialization (parse_xml() and add_xml_to_node()).
`XmlParserMixin` contains the code which does the actual serialization and
deserialization.
This attribute is already deprecated for XBlocks in favour of directly
retrieving it like `block.scope_ids.usage_id.context_key`.
This commit also removes some redundant logging code which was omitted in the
Datadog removal in #19420.
* fix: hide discussion xblock for openedx provider
* fix: test cases
* refactor: hide xblock is provider not legacy and add message in studio
* fix: lint issue
* refactor: update discussion block studio msg
* fix: test discussion xblock query number
JIRA: https://openedx.atlassian.net/browse/BOM-2580
This PR aims at refactoring the discussion xblock sub project and moving it within the xmodule directory effectively removing its position as a sub project within edx-platform
