lacp: Remove enabled flag.
The enabled flag in the LACP module was only used to set the
Collecting and Distributing flags in the LACP protocol. It was
intended to be set by the bonding code to mimic its enabled flag.
The spec is relatively vague on the precise meaning of these flags,
and most implementations do something completely different with
them. For these reasons, it seems acceptable to remove the enabled
flag for the sake of simplicity. A slave is now Collecting and
Distributing if it is attached, or LACP couldn't be negotiated.