app/testpmd: revert setting default RSS

app/testpmd: revert setting default RSS

This reverts the patch that enables default RSS action by setting
key=NULL and key_len=0.
In current testpmd implementation a key pointer must exist if
key_len!=0. For example, the following flow rule will cause a
segmentation fault:
flow create 0 <pattern> actions rss queues 0 1 end key_len 40 / end

Fixes: a4391f8bae85 ("app/testpmd: set default RSS key as null")
Cc: stable@dpdk.org
Signed-off-by: Ophir Munk <ophirmu@mellanox.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>