bookmarks: avoid traceback when two pushes race to delete the same bookmark `hg push -f -B remote-only-bookmark` can raise server-side in `bookmarks._del` (specifically in `self._refmap.pop(mark)`), if the remote-only bookmark got deleted concurrently. Fix this by simply not deleting the non-existent bookmark in that case. For avoidance of doubt, refusing to delete a bookmark that doesn't exist when the push starts is taking care of elsewhere; no change of behavior there. Differential Revision: https://phab.mercurial-scm.org/D8124