atm: remove the unused pre_send and send_bh device operations

atm: remove the unused pre_send and send_bh device operations

atmdev_ops::pre_send (a TX pre-processing hook) and ::send_bh (a
bottom-half capable send variant) have no implementation behind them:
no remaining ATM driver sets either, so vcc_sendmsg() always skipped
pre_send and the raw AAL0/AAL5 paths always fell back to ->send().
The drivers that used these hooks were removed with the legacy ATM
adapters.

Drop both operations and the dead branches that tested for them.

Link: https://patch.msgid.link/20260615194416.752559-8-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>