Revert "net: Handle l3mdev in ip_tunnel_init_flow"

This reverts commit ab6c9463 which is
commit db53cd3d upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: Iab816eb27a65b67d3275e1fb00320fc3a15ca868
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>