Drop project deletion schedules

In !167520 (merged), we introduced project_deletion_schedules.

But later we decided that we should use the same table for groups and projects. Ref &15268 (comment 2453503982).

Implementation guide

1. Create a migration that drops the project_deletion_schedules table.
2. Delete app/models/projects/deletion_schedule.rb
3. Delete db/docs/project_deletion_schedules.yml
4. Remove user association in https://gitlab.com/gitlab-org/gitlab/blob/5400cd53c7f16ce9d0637cf9906a4256e25029f8/app/models/user.rb#L215