ada: Simplify Note_Uplevel_Bound procedure

The procedure Note_Uplevel_Bound was implemented as a custom expression
tree walk. This change replaces this custom tree traversal by a more
idiomatic use of Traverse_Proc.

gcc/ada/

	* exp_unst.adb (Check_Static_Type::Note_Uplevel_Bound): Refactor
	to use the generic Traverse_Proc.
	(Check_Static_Type): Adjust calls to Note_Uplevel_Bound as the
	previous second parameter was unused, so removed.