netfilter: ebt_stp: Remove unused macro BPDU_TYPE_TCN

BPDU_TYPE_TCN is never used after it was introduced.
So better to remove it.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>