Replace BatchContext when running the detached server

[oyvindeide]

When starting the detached server we use BatchContext to start it and keep track of it, but this should be possible to simplify by using the Scheduler directly: https://github.com/equinor/ert/blob/398ce6ad2861eb3489dfce7965e58df30d0d2379/src/everest/detached/init.py#L102

This means we dont need a few ErtConfig for the server, and when running the detached server we dont need to create a run path in the same way as before.