Remove "fedora-branched-*" CI targets

https://hub.docker.com/_/fedora/?tab=tags shows that the "branched" tag was last updated a year ago.

https://github.com/docker-library/official-images/commit/4881e40863a194c0e2c89703aa53cf767ef69966
gives some further hints that the image tag "branched" hasn't been updated
since 4-May-2018.