overseer: remove connect/disconnect state of the Handle

[drahnr]

The existance of this state is merely due to a cyclic dependency that currently exists. It does not represent the state of the overseer.

The chain selection is currently built atop of that, but it should really only consider a is_relay_chain: bool which is based on the known chains that indeed are relay chains.

#3775 introduces a way to avoid breaking the cycle for subsystems, but also lays the ground work to break non-subsystem cycles, yet requires #3773 to land before this can be done cleanly.