netlink: abolish check_config and output_config

Apparently, a long time ago, MSG5 and MSG6 used to contain some
netlink configuration data, which the receiver of the MSG5 or MSG6
would check.

However, for a long time now the output_config function has been a
no-op and the check function has unconditionally eaten and discarded
anything extra in the message.

Furthermore, because the MSG6 is not retransmitted, this mechanism
couldn't be reliable without a protocol change.  So the existing
interface is defective.

So, abolish it the interface, the dummy implementation, and all the
call sites.  The check_config call sites in site.c now instead
directly discard any unexpected data at the end of MSG5 and MSG6.

This patch should cause no behavioural change in actual operation.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>