DAL/Node: remove ready context

What

Move Node_context.ready_ctxt inside Node_context.t.

On top of !14158 (merged).

Why

Given that now all protocols have a DAL plugin (except Genesis), there is no need to wait for a level at which there is a DAL plugin. This also makes the node initialization clearer.

Note: the DAL node could wait that the node is at level at least 2 so that a protocol with a DAL plugin is activated, this is not done, as it is assumed it will be the case.

How

Just refactoring.

Manually testing the MR

CI should suffice.