conntrack: Fix alg expectation cleanup.

conntrack: Fix alg expectation cleanup.

Presently, alg expectations are removed by being time expired.
This was intended to happen before the control connections and
was intended to minimize the extra work involved for tracking and
removing the expectations.  This is not the best option since it
should be possible to remove expectations when a control connection
is removed and a new api is in the works to do this. Also, conceptually
an expectation should not exist without a control connection context
and it can be argued that this should be a strict requirement.

The approach is changed to remove the expectations when the control
connections are removed.  The previous code to expire the expectations
is removed at the same time.

Fixes: bd5e81a0e ("Userspace Datapath: Add ALG infra and FTP.")
Reported-at: https://mail.openvswitch.org/pipermail/ovs-dev/2017-December/341683.html
Signed-off-by: Darrell Ball <dlu998@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>