What you have done here is create an ordinary local branch named origin/feature1.  Git is perfectly happy with this—internally, its name is refs/heads/origin/feature1 which clearly marks it as an ordinary local branch—even though it's terribly confusing to users, who see it as looking like a remote-tracking branch.
As Rob already answered, you can simply delete the local branch with the bogus name.  Alternatively, you can rename it, which avoids having to get off it first:
$ git branch
  master
* origin/feature1
$ git branch -m feature1
$ git branch
  master
* feature1
Note that actual remote branches have full internal names that start with refs/remotes/, and you can run git symbolic-ref HEAD to see the full internal name of the current branch (which may be less confusing, provided you know about the refs/heads/ vs refs/remotes/ thing).