py3: don't return the revid as unicode in hgext/convert/subversion.py

I tried digging why u'' was added in first place, and I was unable to found
something relevant. This might be because some API's takes unicodes, I am not
sure.

Differential Revision: https://phab.mercurial-scm.org/D4454

hgext/convert/subversion.py

@@ -1270,7 +1270,7 b' class svn_sink(converter_sink, commandli'
         self.childmap[parent] = child
 
     def revid(self, rev):
-        return u"svn:%s@%s" % (self.uuid, rev)
+        return "svn:%s@%s" % (self.uuid, rev)
 
     def putcommit(self, files, copies, parents, commit, source, revmap, full,
                   cleanp2):