Gitea and Forgejo chose to implement wiki branch naming differently, but
Forgejo picked the Gitea migration anyway, resulting in an unused column
in the database, which wasn't part of the `Repository` struct either -
something warned about during startup, too.
Similarly, Forgejo chose not to implement User badges at all - but kept
the existing code for it -, and the `badge` table ended up with an
unused `slug` column due to a Gitea migration, and resulted in another
warning at startup.
To keep the database consistent with the code, and to get rid of these
warnings, lets introduce a new migration, which simply drops these
Gitea-specific columns from the database.
Fixes #3463.
Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>
