ada: Fix bug in Sem_Util.Enclosing_Declaration

Fix Sem_Util.Enclosing_Declaration to not return an N_Subprogram_Specification
node. Remove code in various places that was formerly needed to cope with this
misbehavior.

gcc/ada/

	* sem_util.adb (Enclosing_Declaration): Instead of returning a
	subprogram specification node, return its parent (which is
	presumably a subprogram declaration).
	* contracts.adb (Insert_Stable_Property_Check): Remove code
	formerly needed to compensate for incorrect behavior of
	Sem_Util.Enclosing_Declaration.
	* exp_attr.adb (In_Available_Context): Remove code formerly needed
	to compensate for incorrect behavior of
	Sem_Util.Enclosing_Declaration.
	* sem_ch8.adb (Is_Actual_Subp_Of_Inst): Remove code formerly
	needed to compensate for incorrect behavior of
	Sem_Util.Enclosing_Declaration.