util: move "default" lookupreg to posix.py

There is a lookupreg in win32.py, which we get via windows.py

mercurial/posix.py

@@ -338,3 +338,5 b' def unlinkpath(f):'
     except OSError:
         pass
 
+def lookupreg(key, name=None, scope=None):
+    return None

mercurial/util.py

@@ -475,9 +475,6 b' def checkwinfilename(path):'
             return _("filename ends with '%s', which is not allowed "
                      "on Windows") % t
 
-def lookupreg(key, name=None, scope=None):
-    return None
-
 def hidewindow():
     """Hide current shell window.