rtnl_net_debug: Remove rtnl_net_debug_exit().

kernel test robot reported section mismatch in rtnl_net_debug_exit().

  WARNING: modpost: vmlinux: section mismatch in reference: rtnl_net_debug_exit+0x20 (section: .exit.text) -> rtnl_net_debug_net_ops (section: .init.data)

rtnl_net_debug_exit() uses rtnl_net_debug_net_ops() that is annotated
as __net_initdata, but this file is always built-in.

Let's remove rtnl_net_debug_exit().

Fixes: 03fa534 ("rtnetlink: Add ASSERT_RTNL_NET() placeholder for netdev notifier.")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202410101854.i0vQCaDz-lkp@intel.com/
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20241010172433.67694-1-kuniyu@amazon.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/core/rtnl_net_debug.c

@@ -122,10 +122,4 @@ static int __init rtnl_net_debug_init(void)
 	return ret;
 }
 
-static void __exit rtnl_net_debug_exit(void)
-{
-	unregister_netdevice_notifier(&rtnl_net_debug_block);
-	unregister_pernet_device(&rtnl_net_debug_net_ops);
-}
-
 subsys_initcall(rtnl_net_debug_init);