net/mlx5e: Remove unused tstamp SQ field

Remove tstamp pointer in mlx5e_txqsq as it's no longer used after
commit 7c39afb3

 ("net/mlx5: PTP code migration to driver core section").

Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Aya Levin <ayal@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>