net: netlink: Remove unused function

lockdep_genl_is_held() and its caller arm not used now, just remove them.

Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
Link: https://lore.kernel.org/r/20210729074854.8968-1-yajun.deng@linux.dev


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