collab: Drop `channel_messages` table and its dependents (#42392)
This PR drops the `channel_messages` table and its
dependents—`channel_message_mentions` and `observed_channel_messages`—as
they are no longer used.
Release Notes:
- N/A
11b38db3e3Marshall Bowers committed on 11/10/2025, 9:59:05 PM· committed by GitHubparent112b5c1