convert: drop unused getheads from sinks

hgext/convert/common.py

@@ -192,10 +192,6 b' class converter_sink(object):'
         self.path = path
         self.created = []
 
-    def getheads(self):
-        """Return a list of this repository's heads"""
-        raise NotImplementedError
-
     def revmapfile(self):
         """Path to a file that will contain lines
         source_rev_id sink_rev_id

hgext/convert/hg.py

@@ -78,10 +78,6 b' class mercurial_sink(converter_sink):'
     def authorfile(self):
         return self.repo.join("authormap")
 
-    def getheads(self):
-        h = self.repo.changelog.heads()
-        return [hex(x) for x in h]
-
     def setbranch(self, branch, pbranches):
         if not self.clonebranches:
             return