Remove legacy functions used for SteadyState
Created by: PhilipVinc
Those things where still around from netket 2.5 and I had never removed them because one function was still used in the gradient of Lindblad squared.
I now moved the body of the only used function there, and removed the old code that is not used anywhere.
Lastly, this also switches to a new logic inside of linblad, that i had never turned on for some reason, which uses nkjax.vjp instead of nkjax.grad. This makes the density matrix code work with non-holomorphic density matrix ansatze