net/x25: Remove unused x25_terminate_link()

x25_terminate_link() has been unused since the last use was removed
in 2020 by:
commit 7eed751b ("net/x25: handle additional netdev events")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Acked-by: Martin Schiller <ms@dev.tdt.de>
Link: https://patch.msgid.link/20250712205759.278777-1-linux@treblig.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>