Remove unused parts of code.

- in directory src/coupling
- remove EquationNothing class
- remove unused class members and other short parts of unused code
Remove unused parts of code.

- in directory src/fem
- remove DofHandler class
- remove other short parts of unused code
Remove unused parts of code.

- in directory src/fields
- remove unused methods and class members
- remove unused development messages etc.
Remove unused parts of code.

- in directory src/flow
- remove unused methods of DarcyFlow
- remove unused class members and other short parts of unused code
Remove unused parts of code.

- in directory src/input
- remove unused class members, development messages etc.
Remove unused parts of code.

- in directory src/interpolation
- remove parts of code in Interpolant
Remove unused parts of code.

- in directory src/io
- remove unused methods in OutputTime
- remove unused class members and other short parts of unused code
Remove unused parts of code.

- in directory src/la
- remove unused class members and other short parts of unused code
Remove unused parts of code.

- in directory src/mesh/ngh
- remove unused methods and class members
- remove other short parts of unused code
Remove unused parts of code.

- in directory src/mesh
- remove file nodes.cc
- remove unused methods and class members
- remove other short parts of unused code
Remove unused parts of code.

- in directory src/reaction
- remove unused methods, class members and parts of old code
- remove unused debug messages
Remove unused parts of code.

- in directory src/semchem
- remove unused debug messages, variables and other short parts of
unused code
Remove unused parts of code.

- in src/main.cc
Remove src/dead_code directory.