Revert "net: sit: fix memory leak in sit_init_net()"

This reverts commit 375d6d45 which is
commit 07f12b26e21ab359261bf75cfcb424fdc7daeb6d upstream.

Unnecessarily calling free_netdev() from sit_init_net().
ipip6_dev_free() of 4.9.y called free_netdev(), so no need
to call again after ipip6_dev_free().

Cc: Mao Wenan <maowenan@huawei.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Ajay Kaher <akaher@vmware.com>
Reviewed-by: Mao Wenan <maowenan@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>