refactor: remove GitLab API internal polling

What does this MR do?

Remove polling from Pages implemented in !304 (merged) (merged) (merged)

auto_source_gitlab tests are being removed in !483 (closed) but they are failing without API polling so I'm removing them here.
Everything else is just a revert of !304 (merged)
backoff is also removed since it was introduced by !307 (merged) for exponential back-off.

Closes #449 (closed)

TODO

• Added the Changelog (e.g. Changelog: feature) trailer to the commits that need to be included in the changelog
• Added unit tests (if required)
• Added acceptance tests (if required)
• Documentation (if required)
• Code review guidelines
• Go Style guidelines