Instead of marking a device inactive, remove it from the cache entirely.

Flexfiles has a way to report errors back to the server, so we don't want
to stop devices from being tried again for 120 seconds.
Signed-off-by: NWeston Andros Adamson <dros@primarydata.com>
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>