tag.h: Remove unused signature field

Its documented as unused.  So lets just drop it from the structure
since we haven't ever used it.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

tag.h

@@ -9,7 +9,6 @@ struct tag {
 	struct object object;
 	struct object *tagged;
 	char *tag;
-	char *signature; /* not actually implemented */
 };
 
 extern struct tag *lookup_tag(const unsigned char *sha1);