chore: remove forum v1 pin api support

[taimoor-ahmed-1]

Description

This pull request removes legacy code related to the “pin” api functionality within the discussion forum v1 app. The rationale behind this change is to clean up deprecated or unused code paths, thereby simplifying the codebase and reducing potential maintenance overhead.

Implications for users:
• There should be no impact on end-users, as the removed code pertains to deprecated functionality that is no longer in active use.
• Developers working on the discussion forum module will benefit from a cleaner codebase, making future enhancements and maintenance more straightforward.

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[ormsbee]

I've merged this with a different commit message. Please mark breaking changes with "feat!" instead of "chore"

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX staging environment in preparation for a release to production.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.

[pdpinch]

Does this mean that pinning posts on courses that are still using Forums v1 will not work?