net: em_canid: remove useless statements from em_canid_change

tcf_ematch is allocated by kzalloc in function tcf_em_tree_validate(),
so cm_old is always NULL.

Signed-off-by: Duan Jiong <duanj.fnst@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>