Remove bridge code for auto-disabling webhooks after required stop in 18.2

About

!166329 (merged) introduced some bridge code.

As part of the next required stop we can remove the bridge code. The next required stop will be in 18.2.

Implementation

Search the three instances where this issue (https://gitlab.com/gitlab-org/gitlab/-/issues/525446) is mentioned in the codebase and make the removals mentioned.