environment: drop comment_line_char compatibility macro

There is no longer any code which references the single-byte
comment_line_char. Let's drop it, clearing the way for true multi-byte
entries in comment_line_str.

It's possible there are topics in flight that have added new references
to comment_line_char. But we would prefer to fail compilation (and then
fix it) upon merging with this, rather than have them quietly ignore the
bytes after the first.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

environment.h

@@ -8,7 +8,6 @@ struct strvec;
  * The character that begins a commented line in user-editable file
  * that is subject to stripspace.
  */
-#define comment_line_char (comment_line_str[0])
 extern const char *comment_line_str;
 extern int auto_comment_line_char;