update field solver according to master changes

src/PartBunch/FieldSolver.hpp

@@ -27,10 +27,6 @@ template <typename T = double, unsigned Dim = 3>
 using OpenSolver_t =
     ConditionalType<Dim == 3, ippl::FFTOpenPoissonSolver<VField_t<T, Dim>, Field_t<Dim>>>;
 
-template <typename T = double, unsigned Dim = 3>
-using Solver_t = VariantFromConditionalTypes<
-    CGSolver_t<T, Dim>, FFTSolver_t<T, Dim>, P3MSolver_t<T, Dim>, OpenSolver_t<T, Dim>>;
-
 // Define the FieldSolver class
 template <typename T, unsigned Dim>
 class FieldSolver : public ippl::FieldSolverBase<T, Dim> {