ada: Small cleanup in expansion of array aggregates in allocators

Convert_Array_Aggr_In_Allocator does nothing that Late_Expansion cannot do,
so this deletes the former and moves its support code for Storage_Model to
the latter.  No functional changes.

gcc/ada/ChangeLog:

	* exp_aggr.adb (Convert_Array_Aggr_In_Allocator): Delete.
	(Convert_Aggr_In_Allocator): Do not call above procedure.
	(Late_Expansion): Deal with a target that is the dereference of a
	prefix with a Storage_Model.  Remove an useless actual parameter
	in the call to Build_Array_Aggr_Code.