shallow: remove unused #include "sigchain.h"

After switching to use the tempfile module in commit 6e122b44
(setup_temporary_shallow(): 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>

shallow.c

@@ -10,7 +10,6 @@
 #include "diff.h"
 #include "revision.h"
 #include "commit-slab.h"
-#include "sigchain.h"
 
 static int is_shallow = -1;
 static struct stat_validity shallow_stat;