ada: Cleanup calculation of task stacks

Code cleanup; semantics is unaffected.

gcc/ada/

	* exp_ch3.adb (Count_Default_Sized_Task_Stacks): Do not look for
	tasks inside record discriminants; remove avoid repeated call to
	Has_Task that happened for record components.
	(Expand_N_Object_Declaration): Use high-level routine to detect
	array types and subtypes; remove unused initial values.