rpc: Remove keepalive_required option
    
    Since its introduction in 2011 (particularly in commit f4324e329275),
    the option doesn't work.  It just effectively disables all incoming
    connections.  That's because the client private data that contain the
    'keepalive_supported' boolean, are initialized to zeroes so the bool is
    false and the only other place where the bool is used is when checking
    whether the client supports keepalive.  Thus, according to the server,
    no client supports keepalive.
    
    Removing this instead of fixing it is better because a) apparently
    nobody ever tried it since 2011 (4 years without one month) and b) we
    cannot know whether the client supports keepalive until we get a ping or
    pong keepalive packet.  And that won't happen until after we dispatched
    the ConnectOpen call.
    
    Another two reasons would be c) the keepalive_required was tracked on
    the server level, but keepalive_supported was in private data of the
    client as well as the check that was made in the remote layer, thus
    making all other instances of virNetServer miss this feature unless they
    all implemented it for themselves and d) we can always add it back in
    case there is a request and a use-case for it.
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>