Removed dormant discussion items

What does this MR do?

Removing dormant code related to app/view/discussions

Screenshots

NA until the pipline passes 100%

Does this MR meet the acceptance criteria?

Conformity

TO BE DETERMINED, removing dormant code wont have any visual impact. Once all items have been successfully removed the change log can be updated.

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #204739 (closed)