scmutil: remove deprecated revpairnodes method (API)

scmutil: remove deprecated revpairnodes method (API)

--- a/mercurial/scmutil.py	Fri May 11 00:54:37 2018 -0400
+++ b/mercurial/scmutil.py	Fri May 11 00:55:01 2018 -0400
@@ -565,11 +565,6 @@
     tree = revsetlang.parse(revspec)
     return tree and tree[0] in ('range', 'rangepre', 'rangepost', 'rangeall')
 
-def revpairnodes(repo, revs):
-    repo.ui.deprecwarn("revpairnodes is deprecated, please use revpair", "4.6")
-    ctx1, ctx2 = revpair(repo, revs)
-    return ctx1.node(), ctx2.node()
-
 def revpair(repo, revs):
     if not revs:
         return repo['.'], repo[None]