Ticket #2586: broken extended keybindings in editor.

ctrl-x shortcut is used in default keymap twice:
  * as shotrcut for WordRight action (see 0908c8baea7f85aab153fbd815fda7fdfdf6bb6d);
  * as prefix for extended keybindings.

To fix the second issue, shortcut for editor.ExtendedKeyMap is
unassigned now because extended shortcuts are unused in editor in
default keymap.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>

misc/mc.keymap.default

@@ -331,7 +331,7 @@ SelectCodepage = alt-e
 Options =
 OptionsSaveMode =
 LearnKeys =
-ExtendedKeyMap = ctrl-x
+ExtendedKeyMap =
 
 [viewer]
 Help = f1