doc: remove obsolete vector Tx explanations from mlx5 guide

doc: remove obsolete vector Tx explanations from mlx5 guide

Vectorized routines were removed in result of Tx datapath refactoring,
and devarg keys documentation was updated.

However, more updating should have been done. In environment variables
doc, there was explanation according to vectorized Tx which isn't
relevant anymore.

This patch removes this irrelevant explanation.

Fixes: a6bd4911ad93 ("net/mlx5: remove Tx implementation")
Cc: stable@dpdk.org
Signed-off-by: Michael Baum <michaelba@nvidia.com>
Reviewed-by: Raslan Darawsheh <rasland@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>