MB-12179: Remove ep_io_* stats

We are moving these stats from stats all to the kvstore instances
in ep-engine.

Change-Id: Ibb3dff643da5850218b018897b38988e4303f881
Reviewed-on: http://review.couchbase.org/42050
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>