net/sched: act_ct: Remove redundant ct get and check

The assignment is not being used and redundant.
The check for null is redundant as nf_conntrack_put() also
checks this.

Signed-off-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Paul Blakey <paulb@nvidia.com>
Link: https://lore.kernel.org/r/20210428060532.3330974-1-roid@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>