net/tls: remove old exports of sk_destruct functions

tls_device_sk_destruct being set on a socket used to indicate
that socket is a kTLS device one.  That is no longer true -
now we use sk_validate_xmit_skb pointer for that purpose.
Remove the export.  tls_device_attach() needs to be moved.

While at it, remove the dead declaration of tls_sk_destruct().

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>