op-supervisor: Drop multi-node support

[sebastianst]

The supervisor currently has (some) multi-node support, to drive multiple managed nodes on the same chain. But it's not fully tested or supported yet. Recently #16008 had to skip the TestMultiNode test in op-e2e/interop/interop_test.go since it kept failing and it was decided that this is low priority at the moment because the future of multi-node support isn't clear.

We should decide to either fully invest in it, or drop it completely. Edit: we have decided to drop multi-node support for now. This ticket now serves as the tracking ticket for multi-node support removal.

[tynes]

We can drop support completely for now and re-introduce it in a future release