ada: Rely on default parameter when making non-constant nodes

When calling Make_Access_To_Object_Definition and Make_Object_Declaration we
can rely on the default value of parameter Constant_Present being False. This
makes code cleaner and consistent with relying on the default value of other
parameters of this routine, like Null_Exclusion_Present.

Code cleanup; semantics is unaffected.

gcc/ada/ChangeLog:

	* exp_ch3.adb, exp_disp.adb: Remove explicit actual parameter
	Constant_Present equal False; tune comments.