Code cleanup related to itypes for Ada 2022 null array aggregates.
Remove routine that was added in 2011 but is not referenced by
GNATprove since 2015.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sem_util.ads (Itype_Has_Declaration): Remove spec.
	* sem_util.adb (Itype_Has_Declaration): Remove body.