Don't print the command name for diskinfo in cbstats

This was likely left in on accident as a debug statement. We don't
print the command for any other stats types and we shouldn't do
it for diskinfo either.

Change-Id: Ied9d55f8cb4d2d4d23e2812f1335c42ec3aea542
Reviewed-on: http://review.couchbase.org/37138
Reviewed-by: David Liao <david.liao@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>