app/testpmd: revert setting MTU explicitly after configure

app/testpmd: revert setting MTU explicitly after configure

Setting MTU after each 'rte_eth_dev_configure()' prevents using
"--max-pkt-len=N" parameter and "port config all max-pkt-len #" command

This is breaking DTS scatter test case which is using
"--max-pkt-len=9000" testpmd parameter.

Reverting workaround to recover the DTS testcase.

Fixes: 1c21ee95cf52 ("app/testpmd: fix MTU after device configure")
Cc: stable@dpdk.org
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Tested-by: Bo Chen <box.c.chen@intel.com>