bundle2: remove _getbundleextrapart

bundle2: remove _getbundleextrapart

Extensions can now add new part generators with the getbundle2partsgenerator
decorator instead.

--- a/mercurial/exchange.py	Fri Oct 17 12:34:27 2014 -0500
+++ b/mercurial/exchange.py	Thu Sep 25 11:53:28 2014 +0900
@@ -1206,12 +1206,6 @@
         markers = repo.obsstore.relevantmarkers(subset)
         buildobsmarkerspart(bundler, markers)
 
-@getbundle2partsgenerator('extra')
-def _getbundleextrapart(bundler, repo, source, bundlecaps=None,
-                        b2caps=None, **kwargs):
-    """hook function to let extensions add parts to the requested bundle"""
-    pass
-
 def check_heads(repo, their_heads, context):
     """check if the heads of a repo have been modified