campaigns: Remove automatic webhook syncing for Bitbucket Server

Created by: ryanslade

This change removes our automatic syncing of webhook configuration with our Bitbucket Server plugin.

After running into numerous issues and questions from customers we decided that the complexity it adds was not worth it.

If needed, this change can be reverted at a later date to re-enable the feature. However, note that our currently deployed version (3.16) has a bug which was fixed here but never made into an official release.

This means that it is possible that existing customers who have configured more than one external service pointing at the same BBS instance will only have one webhook configured. If this feature is re-enabled we'll need to ensure that we delete those webhooks.

Options for how to do this were discussed here