exchange: remove duplicated addition to pushop.stepdone

Not sure how it got there but it is useless.

mercurial/exchange.py

@@ -373,7 +373,6 b' def _pushb2ctx(pushop, bundler):'
         return
     pushop.stepsdone.add('changesets')
     # Send known heads to the server for race detection.
-    pushop.stepsdone.add('changesets')
     if not _pushcheckoutgoing(pushop):
         return
     pushop.repo.prepushoutgoinghooks(pushop.repo,