Cells: Iteration 1: Remove `broadcast_messages.namespace_id` database column

Problem

The broadcast_messages.namespace_id was meant to bring broadcast_messages feature to group-level and allowing notify all users. This column was never used.

This feature won't work as originally intended with the new cells architecture.

This issue is to remove the broadcast_messages.namespace_id column. There is a documented procedure to remove columns.

Removing columns is tricky because running GitLab processes may still be using the columns. To work around this safely, you need three steps in three releases:

1. Ignoring the column (release M)
2. Dropping the column (release M+1)
3. Removing the ignore rule (release M+2)

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Another issue has been created to replace the group broadcast message feature with an organization broadcast message.