exo-utils no longer needed

OK, more GTK2-era things are spotted. exo-utils.{c,h} contains variety of exo_noops for a reason that before GTK 2.10, GTK did not support NULLable for function pointer arguments. A comment on xfce4-places-plugin explains about this. It is not the case these days since XFCE migrated to GTK3.

MR for deprecating this will be posted soon.

Projects that still use this function: Thunar, Exo, xfce4-places-plugin