newsticker: Change wording of confirmation prompt regarding obsolete variable `newsticker-groups-filename'.

Fixes: debbugs:19165

* lisp/net/newst-treeview.el (newsticker-groups-filename): Change
default value to nil.  Point out that variable is obsolete in doc
string.
(newsticker--treeview-load): Change wording of the questions the
user is asked when `newsticker-groups-filename' is found to be
used and we offer to read and remove the groups file.
Use `mouse-menu-bar-map' when no other menus are available.

* lisp/menu-bar.el (menu-bar-open): When everything else fails,
use (mouse-menu-bar-map).