ada: Cleanup clearing flags on package variables

ada: Cleanup clearing flags on package variables

When killing flags on assignable entities we iterated from First_Entity
and then again from First_Private_Entity. This second iteration was
unnecessary, because the entity chain that starts with First_Entity
contains all entities, including the private ones.

This is just a performance improvement; the behavior is unchanged.

gcc/ada/

	* sem_ch7.adb (Clear_Constants): Only iterate from First_Entity
	through Next_Entity; only examine variables because packages have
	no assignable formal parameters.