atm: remove the unused send_oam / push_oam callbacks

atm: remove the unused send_oam / push_oam callbacks

The atmdev_ops::send_oam device operation and the atm_vcc::push_oam
callback were the kernel's interface for raw F4/F5 OAM cell exchange.
Nothing assigns them a non-NULL value and nothing ever invokes them:
the core only ever initialises push_oam to NULL (in vcc_create() and the
AAL init helpers) and the Solos driver only lists send_oam = NULL for
documentation. The drivers that actually drove OAM through these hooks
were removed along with the legacy ATM adapters.

Drop both callbacks and the NULL initialisers.

Link: https://patch.msgid.link/20260615194416.752559-3-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>