refactor: remove Yafc.Model.SolverHelper.cs (shpaass#288)

The `SolverHelper` class looked like an answer to the question we're not
aware of yet, but it wasn't used anywhere, so I asked ShadowTheAge about
it.

> ShadowTheAge: Probably some attempt to streamline ortools solver
creation. If it is not used then probably failed.

Fixes shpaass#285