read-cache: remove unused #include "sigchain.h"

After switching to use the tempfile module in commit f6ecc62d
(write_shared_index(): use tempfile module), no declarations from
sigchain.h are used in read-cache.c anymore. Thus, remove the #include.
Signed-off-by: NTobias Klauser <tklauser@distanz.ch>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

read-cache.c

@@ -17,7 +17,6 @@
 #include "strbuf.h"
 #include "varint.h"
 #include "split-index.h"
-#include "sigchain.h"
 #include "utf8.h"
 
 static struct cache_entry *refresh_cache_entry(struct cache_entry *ce,