net: Remove ->exit_batch_rtnl().

There are no ->exit_batch_rtnl() users remaining.

Let's remove the hook.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/20250411205258.63164-15-kuniyu@amazon.com


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