revision.c: remove unneeded check for NULL

The function is called only from one place, which makes sure to have
`interesting_cache` not NULL.  Additionally the variable is a
dereferenced a few lines before unconditionally, which would have
resulted in a segmentation fault before hitting this check.

Signed-off-by: Stefan Beller <sbeller@google.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.c

@@ -350,7 +350,7 @@ static int everybody_uninteresting(struct commit_list *orig,
 		list = list->next;
 		if (commit->object.flags & UNINTERESTING)
 			continue;
-		if (interesting_cache)
+
 		*interesting_cache = commit;
 		return 0;
 	}