There is GtkSettings:gtk-enable-mnemonics: now, so there is no
reason to do the same in GIMP:

* app/config/gimpguiconfig.[ch]: turn "menu-mnemonics" into a dummy.

* app/dialogs/preferences-dialog.c: remove its GUI.

* app/widgets/gimpactionfactory.[ch]
* app/widgets/gimpactiongroup.[ch]: remove infrastructure for disabling
  menu mnemonics.

* app/actions/actions.c: bye bye glue code.