pimd: drop printing IP_MULTICAST_LOOP sockopt

pimd: drop printing IP_MULTICAST_LOOP sockopt

This isn't a system or interface setting, it's a socket behavior.  It is
both irrelevant and confusing to the user since it doesn't affect any
system behavior (but it sounds like it does).  Whether it is enabled or
not is solely relevant to how the code is designed to work.

So, remove it from output.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>