[Ada] Clean up scanner

[Ada] Clean up scanner

This patch removes some obsolete code in the scanner and related files,
and corrects some comments. Tok_Special is used only by the
preprocessor, and uses only the two characters '#' and '$'.

It might be simpler to have a single flag indicating we're scanning for
preprocessing, instead of the Special_Characters array and the
End_Of_Line_Is_Token flag, but that's for another day.

gcc/ada/

	* scans.ads: Fix obsolete comments about Tok_Special, and give
	Special_Character a predicate assuring it is one of the two
	characters used in preprocessing.
	* scng.ads: Clean up comments.
	* scng.adb: Clean up handling of Tok_Special.  Remove comment
	about '@' (target_name), which doesn't seem very helpful.
	Set_Special_Character will now blow up if given anything other
	than '#' and '$', because of the predicate on Special_Character;
	it's not clear why it used to say "when others => null;".
	Remove Comment_Is_Token, which is not used.
	* scn.ads: Remove commented-out use clause.  Remove redundant
	comment.
	* ali-util.adb: Use "is null" for do-nothing procedures.
	* gprep.adb (Post_Scan): Use "is null".