Get rid of a //-style comment.

Not sure how "//XXX" got into a committed patch in the first place,
as it's both content-free and against project style.  pgindent made a
bit of a hash of it, too.

Going forward, we should have at least one buildfarm member using
"gcc -ansi" to catch such things, at least till such time as we
decide the project target language isn't C90 any more.  I've turned
this option on on dromedary.

src/test/regress/pg_regress.c

@@ -1974,7 +1974,6 @@ help(void)
 	printf(_("                            (can be used multiple times to concatenate)\n"));
 	printf(_("  --temp-instance=DIR       create a temporary instance in DIR\n"));
 	printf(_("  --use-existing            use an existing installation\n"));
-	//XXX
 	printf(_("\n"));
 	printf(_("Options for \"temp-instance\" mode:\n"));
 	printf(_("  --no-locale               use C locale\n"));