Do not try do delete `testing` branch anymore

[rudymatela]

In #134, we transitioned from testing on testing/<id> instead of just testing. In order to maintain backwards compatibility, we had to delete testing every time we created a new testing/<id> branch.

That has been a month ago, and most projects that use Hoff should have had at least one PR triggering the deletion of testing. So we should remove this code that tries deletion every time.

We currently see the following in the logs every time we merge:

Aug 18 13:37:12 hoff hoff[4321]: [Warn] error: git push -d failed. Reason: error: unable to delete 'testing': remote ref does not exist
Aug 18 13:37:30 hoff hoff[4321]: error: failed to push some refs to 'git@github.com:<org>/<repo>.git'

This should get rid of entries like the above.

TODO

• [x] check that all projects/repos do not have a testing branch anymore
  • [x] delete any found
• [x] remove the testing deletion code: #173
• [ ] double-check that all projects/repos do not have a testing branch anymore

[rudymatela]

Thanks to @tomwassenberg for helping me with the deletion in some repos! :smile: :tada: