Revert "Add new @ shortcut for HEAD"

This reverts commit cdfd94837b27c220f70f032b596ea993d195488f, as it
does not just apply to "@" (and forms with modifiers like @{u}
applied to it), but also affects e.g. "refs/heads/@/foo", which it
shouldn't.

The basic idea of giving a short-hand might be good, and the topic
can be retried later, but let's revert to avoid affecting existing
use cases for now for the upcoming release.
Merge branch 'master' into next

* master:
  Revert "Add new @ shortcut for HEAD"