Ignore failures from removing multicast addresses from the parent (trunk)

Ignore failures from removing multicast addresses from the parent (trunk)
interface when tearing down a vlan interface.  If a trunk interface is
detached, all of its multicast addresses are removed before the ifnet
departure eventhandlers are invoked.  This means that all of the multicast
addresses are removed before the vlan interfaces are removed which causes
the if_delmulti() calls in the vlan teardown to fail.

In the VLAN_ARRAY case, this left vlan interfaces referencing a no longer
valid parent interface.  In the !VLAN_ARRAY case, the eventhandler gets
stuck in an infinite loop retrying vlan_unconfig_locked() forever.  In
general the callers of vlan_unconfig_locked() do not expect nor handle
failure, so I believe it is safer to ignore the errors and tear down as
much of the vlan state as possible.

Silence from:	net@
MFC after:	4 days