debugdiscovery: drop reference to invalid --branch option

It seems like it didn't even exist when debugdiscovery was introduced
in cb98fed52495 (discovery: add new set-based discovery, 2011-05-02).

Differential Revision: https://phab.mercurial-scm.org/D1691

mercurial/debugcommands.py

@@ -744,8 +744,7 b' def debugstate(ui, repo, **opts):'
 def debugdiscovery(ui, repo, remoteurl="default", **opts):
     """runs the changeset discovery protocol in isolation"""
     opts = pycompat.byteskwargs(opts)
-    remoteurl, branches = hg.parseurl(ui.expandpath(remoteurl),
-                                      opts.get('branch'))
+    remoteurl, branches = hg.parseurl(ui.expandpath(remoteurl))
     remote = hg.peer(repo, opts, remoteurl)
     ui.status(_('comparing with %s\n') % util.hidepassword(remoteurl))