net/tls: Remove redundant workqueue flush before destroy

destroy_workqueue() safely destroys the workqueue after draining it.
No need for the explicit call to flush_workqueue(). Remove it.

Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://lore.kernel.org/r/20220801112444.26175-1-tariqt@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>