There is a HEAD set on the bare repo, which actually points to the branch which is checked out when you run git clone <url>
without arguments.
As far as I know you can't delete the remote HEAD branch, so if you want to delete the master you have to push a second branch, move your HEAD to that branch, then push an empty branch in master:
locally:
git branch mybranch
git push origin mybranch
on the bare-repo:
git symbolic-ref HEAD refs/heads/mybranch
locally:
git push origin :master
Of course you can then push another branch you call master
if you want, and move the HEAD back to it.
git push origin newbranch:master
git symbolic-ref HEAD refs/heads/master #(on bare repo)