Merge branch 'feature/fs293-remove-solvers' into 'master'

[linear] remove self-written parallel linear solvers

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.

See merge request !28