In dumux/linear/boxlinearsolver.hh, there are several linear solver backends which use "self-written" infrastructure for parallel runs from several files in dumux/linear.
Deprecate those solvers and the infrastructure files due to the following reasons:
The parallelism is restricted to box.
The iteration counts deteriorate with growing process numbers.
The code is hard to maintain or to generalize.
We have the general / rather robust / mostly upstream-maintained AMGBackend.
Use the corresponding sequential solvers from seqsolverbackend.hh by default and in the serial tests.
This implements FS#293.