cocci: retire is_null_sha1() rule

Since 8d4d86b0 (cache: remove null_sha1, 2019-08-18) removed the
is_null_sha1() function, rewrite rules to correct callers of the
function to use is_null_oid() instead has become irrelevant, as any
new callers of the function will get caught by the compiler much
more quickly without spending cycles on Coccinelle.

Remove these rules.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/coccinelle/object_id.cocci

@@ -1,15 +1,3 @@
-@@
-struct object_id OID;
-@@
-- is_null_sha1(OID.hash)
-+ is_null_oid(&OID)
-
-@@
-struct object_id *OIDPTR;
-@@
-- is_null_sha1(OIDPTR->hash)
-+ is_null_oid(OIDPTR)
-
 @@
 struct object_id OID;
 @@