net/mlx4_en: Remove unused performance counters

Performance analysis counters are maintained under the MLX4_EN_PERF_STAT
definition, which is never set.
Clean them up, with all related structures and logic.

Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Link: https://lore.kernel.org/r/20201118103427.4314-1-tariqt@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>