This file was introduced originally, because the AcceleratedGSStep
class would only accept linear solvers for the linear correction,
but no iteration steps.  The FixedStepLinearSolver could be used
as a shim to practically turn an iteration step into a solver.

Recently, the TNNMGStep class got generalized and now accepts
LinearIterationStep objects directly, in addition to linear solvers.
This makes the FixedStepLinearSolver class obsolete.

If ever somebody else should need it, it should be in dune-solvers
anyway.