l2tp: remove unused tunnel magic field

Since l2tp no longer derives tunnel pointers directly via
sk_user_data, it is no longer useful for l2tp to check tunnel pointers
using a magic feather. Drop the tunnel's magic field.

Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>