ada: Remove guards against traversal of empty list of aspects

ada: Remove guards against traversal of empty list of aspects

When iterating over Aspect_Specifications, we can use First/Next
directly even if the Aspect_Specifications returns a No_List or
the list has no items.

Code cleanup.

gcc/ada/

	* aspects.adb (Copy_Aspects): Style fix.
	* contracts.adb (Analyze_Contracts): Style fix.
	(Save_Global_References_In_Contract): Remove extra guards.
	* par_sco.adb (Traverse_Aspects): Move guard to the caller and
	make it consistent with Save_Global_References_In_Contract.
	* sem_ch12.adb (Has_Contracts): Remove extra guards.
	* sem_ch3.adb (Delayed_Aspect_Present, Get_Partial_View_Aspect,
	Check_Duplicate_Aspects): Likewise.
	* sem_disp.adb (Check_Dispatching_Operation): Likewise.