exchange: remove check for 'format' key

When the 'kwargs' variable was added in 12f161f08d74 (bundle2: allow
pulling changegroups using bundle2, 2014-04-01), it could contain only
'bundlecaps', 'common' and 'heads', so the check for 'format' would
always be false. Since then, _pullbundle2extraprepare() has been added
for hooks, but it seems unlikely that they would a 'format' key.

mercurial/exchange.py

@@ -984,8 +984,6 b' def _pullbundle2(pullop):'
             kwargs['obsmarkers'] = True
             pullop.stepsdone.add('obsmarkers')
     _pullbundle2extraprepare(pullop, kwargs)
-    if kwargs.keys() == ['format']:
-        return # nothing to pull
     bundle = pullop.remote.getbundle('pull', **kwargs)
     try:
         op = bundle2.processbundle(pullop.repo, bundle, pullop.gettransaction)