iocp: remove 'free_bufinfo'

Moving to a read2 model

Change-Id: I363a425ca30417bb0ddd609d1515905bcaf0e2a5
Reviewed-on: http://review.couchbase.org/35879
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>