debugdiscovery: drop reference to non-existent --remote-head 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/D1693

mercurial/debugcommands.py

@@ -782,8 +782,7 b' def debugdiscovery(ui, repo, remoteurl="'
         elif rheads <= common:
             ui.write(("remote is subset\n"))
 
-    remoterevs, _checkout = hg.addbranchrevs(repo, remote, branches,
-                                             opts.get('remote_head'))
+    remoterevs, _checkout = hg.addbranchrevs(repo, remote, branches, revs=None)
     localrevs = opts['rev']
     doit(localrevs, remoterevs)