net/mlx5: Remove a useless 'drain_workqueue()' call in 'mlx5e_ipsec_cleanup()'

'destroy_workqueue()' already calls 'drain_workqueue()', there is no need
to call it explicitly.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>