Remove initialisation of remote channels.

[ablaom]

In MLJBase the __init__ function initialises a remote channel called MACHINE_CHANNEL, which is only used for (important) testing purposes. However, this is an obstacle for certain kinds of scaling; see this discourse thread.

[ablaom]

@OkonSamuel has suggested on slack to make initialisation conditional on a new environment variable, which sounds like a good idea to me.