systat: Chase 70ea484e3ec5, removing vdev_cache

As of 70ea484e3ec5 vdev_cache has been removed. Stop reporting on it.
Without this systat reports:

sysctl(kstat.zfs.misc.vdev_cache_stats.misses...) failed:
No such file or directory

Reviewed by: mav
Differential Revision: https://reviews.freebsd.org/D40742