pppoe: remove kernel-mode relay support

pppoe: remove kernel-mode relay support

The kernel-mode PPPoE relay feature and its two associated ioctls
(PPPOEIOCSFWD and PPPOEIOCDFWD) are not used by any existing userspace
PPPoE implementations. The most commonly-used package, RP-PPPoE [1],
handles the relaying entirely in userspace.

This legacy code has remained in the driver since its introduction in
kernel 2.3.99-pre7 for over two decades, but has served no practical
purpose.

Remove the unused relay code.

[1] https://dianne.skoll.ca/projects/rp-pppoe/

Signed-off-by: Qingfang Deng <dqfext@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Link: https://patch.msgid.link/20260224015053.42472-1-dqfext@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>