compositor: Remove stale prototype
compositor: Remove stale prototype

We got rid of this function.

xorgy/weston,giucam/weston,jonnylamb/weston,kwm81/weston,giucam/weston,udoprog/weston,mchalupa/weston,sir-murray/weston,krezovic/weston,Fantu/compositor-spice,mchalupa/weston,Fantu/compositor-spice,Gnurou/weston,Gnurou/weston,jonnylamb/weston,Fantu/compositor-spice,kwm81/weston,udoprog/weston,xorgy/weston,Tarnyko/weston-xdg_surface_present,eyolfson/weston,sir-murray/weston,eyolfson/weston,krezovic/weston,Tarnyko/weston-xdg_surface_present