gre6: Clean up unused struct ipv6_tel_txoption definition

Commit b05229f4

 ("gre6: Cleanup GREv6 transmit path, call common GRE
functions") removed the ip6gre specific transmit function, but left the
struct ipv6_tel_txoption definition. Clean it up.

Signed-off-by: Jakub Sitnicki <jkbs@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>