desktop: Drop im-config (#152)

This package sets `GTK_IM_MODULE=ibus` which causes inappropriate
position of the IBus candidate window on Wayland.

No packages depend on this so we can safely remove it.

See also:
elementary/gala#2793 (comment)

Author: ryonakano

desktop

@@ -32,7 +32,6 @@ Input methods:
 
  * (gtk-im-libthai)
  * (ibus-chewing)
- * (im-config)
  * (ibus)
  * (ibus-gtk)
  * (ibus-gtk3)