Restore wrong (but performant) possible_watcher impl

Aims to fix https://www.openproject.org/work_packages/15510

The show page of a WorkPackage takes ages to load when there are *many* users in the project. This is because of the slow watcher implementation.
This PR restores the old (crappy) watcher code to make things fast again.

A correct *and* fast implementation is proposed in https://github.com/opf/openproject/pull/1750 -- when this is reviewed and merged, everything is good again.