"error_at_rich_loc" went away in r254280 (in favor of overloading
"error_at"), but there was a stray reference in a comment.

Remove it.

gcc/ChangeLog:
	* gcc-rich-location.h (gcc_rich_location::add_location_if_nearby):
	Fix usage of "error_at_rich_loc" in the comment.

From-SVN: r265177