build: remove useless update-sections gtk-doc target
build: remove useless update-sections gtk-doc target

The file "unused" is generated when symbols are missing. So it's better
to move each symbol from "unused" to the right place in "sections".

GNOME/gtksourceview,cburschka/gtksourceview,cburschka/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,cburschka/gtksourceview,GNOME/gtksourceview,cburschka/gtksourceview,uajain/gtksourceview,uajain/gtksourceview,cburschka/gtksourceview,cburschka/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,uajain/gtksourceview,uajain/gtksourceview,cburschka/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,cburschka/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,cburschka/gtksourceview,GNOME/gtksourceview,cburschka/gtksourceview