[Ada] Remove useless code related to current value propagation

[Ada] Remove useless code related to current value propagation

The current value propagation applies only to assignable objects and
doesn't make sense for subprogram entities. This was a mistake
introduced when extending the current value propagation years ago.

Cleanup related to fixing interference between expansion of attribute
Loop_Entry and current value propagation.

gcc/ada/

	* sem_attr.adb (Address_Checks): Remove call to
	Kill_Current_Values for subprogram entities, because this
	routine only does something for object entities.