Remove checks against DDB, which isn't used in this file.

My intention is to bring no functional change.

Discussion on:	IRC
Reviewed by:	ed, kan, rink,

sys/kern/vfs_mount.c

@@ -68,13 +68,8 @@ __FBSDID("$FreeBSD$");
 #include <security/mac/mac_framework.h>
 
 #include "opt_rootdevname.h"
-#include "opt_ddb.h"
 #include "opt_mac.h"
 
-#ifdef DDB
-#include <ddb/ddb.h>
-#endif
-
 #define	ROOTNAME		"root_device"
 #define	VFS_MOUNTARG_SIZE_MAX	(1024 * 64)