[keyserver] Delete sidebars created from pinned message robotexts

Summary:
Depends on D9229

These threads are causing our keyserver to completely crash and the app to break (ENG-4849). From running the base query on my keyserver, I can see that Jacek is the only user who has threads like these.

The most immediate solution is to delete these test threads. A later diff will prevent their creation.

Test Plan: I repro'd the issue in my local environment following the steps in the linked Linear task's description. After that the web app failed to load. After applying this migration, the web app was once again able to load.

Reviewers: atul, rohan, inka

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D9230