netlink: remove unused 'compare' function

No users in the tree.  Tested with allmodconfig build.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20230308142006.20879-1-fw@strlen.de


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