[Ada] Clean up GNAT.Calendar.Time_IO.Value

[Ada] Clean up GNAT.Calendar.Time_IO.Value

gcc/ada/

	* libgnat/g-catiio.adb (Parse_ISO_8601): Minor cleanups:

	Give some objects clearer names.
	Make some objects more local to where they are used.

	Remove some validity checks that can't fail, because some of
	the variables tested have been moved so they're not visible here.
	Anyway, Wrong_Syntax is about errors in the input, not data
	validity.

	Use Time_Zone_Seen instead of Local_Sign = ' ' to determine
	that a time zone indication was seen.

	We don't need to distinguish two different kinds of
	syntax error (End_Of_Source_Reached and Wrong_Syntax),
	so use Wrong_Syntax always.

	Remove comment, "Certain scanning scenarios may handle
	this exception on their own."; there are no such scenarios.