Deprecate and ignore gtk-enable-mnemonics and gtk-auto-mnemonics" properties

They are both enabled by default.

gdk/x11/gdksettings.c

@@ -58,7 +58,6 @@ static const struct {
   {"Net/FallbackIconTheme",   "gtk-fallback-icon-theme"},
   {"Gtk/TouchscreenMode",     "gtk-touchscreen-mode"},
   {"Gtk/EnableAccels",        "gtk-enable-accels"},
-  {"Gtk/EnableMnemonics",     "gtk-enable-mnemonics"},
   {"Gtk/ScrolledWindowPlacement", "gtk-scrolled-window-placement"},
   {"Gtk/IMModule",            "gtk-im-module"},
   {"Fontconfig/Timestamp",    "gtk-fontconfig-timestamp"},
@@ -66,7 +65,6 @@ static const struct {
   {"Net/EnableInputFeedbackSounds", "gtk-enable-input-feedback-sounds"},
   {"Net/EnableEventSounds",   "gtk-enable-event-sounds"},
   {"Gtk/CursorBlinkTimeout",  "gtk-cursor-blink-timeout"},
-  {"Gtk/AutoMnemonics",       "gtk-auto-mnemonics"},
   {"Gtk/VisibleFocus",        "gtk-visible-focus"},
   {"Gtk/ShellShowsAppMenu",   "gtk-shell-shows-app-menu"},
   {"Gtk/ShellShowsMenubar",   "gtk-shell-shows-menubar"},