app/testpmd: revert max Rx packet length adjustment

app/testpmd: revert max Rx packet length adjustment

The fix of max_rx_pkt_len for allowing VLAN packets in all cases
was breaking configuration of some drivers. Example with virtio:

	Ethdev port_id=0 max_rx_pkt_len 11229 > max valid value 9728
	Fail to configure port 0

Trying to fix the logic was revealing other issues in some drivers.
That's why it is decided to revert.

The workaround for the original issue would be
to set the MTU explicitly from the application
with rte_eth_dev_set_mtu().
See RFC: https://patches.dpdk.org/patch/83756/

Fixes: f6870a7ed6b3 ("app/testpmd: fix max Rx packet length for VLAN packet")
Cc: stable@dpdk.org
Reported-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Lance Richardson <lance.richardson@broadcom.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>