doc: remove queue stats mapping from testpmd guide

doc: remove queue stats mapping from testpmd guide

The "--tx-queue-stats-mapping" and "--rx-queue-stats-mapping"
and display and clear of "stats_map" have been removed from
testpmd.

This patch deletes some descriptions about queue stats mapping
in testpmd doc.

Fixes: 08dcd1870686 ("app/testpmd: fix queue stats mapping configuration")
Cc: stable@dpdk.org
Signed-off-by: Huisong Li <lihuisong@huawei.com>
Signed-off-by: Min Hu (Connor) <humin29@huawei.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>