[cleanup] Remove clutter

exercises/exercise-basic/injection2pproblem.hh

exercises/exercise-biomineralization/biominproblem.hh

...s/exercise-coupling-ff-pm/interface/freeflowsubproblem.hh

...ercise-coupling-ff-pm/interface/porousmediumsubproblem.hh

...ises/exercise-coupling-ff-pm/models/freeflowsubproblem.hh

.../exercise-coupling-ff-pm/models/porousmediumsubproblem.hh

.../exercise-coupling-ff-pm/turbulence/freeflowsubproblem.hh

...rcise-coupling-ff-pm/turbulence/porousmediumsubproblem.hh

exercises/exercise-fluidsystem/2p2cproblem.hh

exercises/exercise-fluidsystem/2pproblem.hh

exercises/exercise-grids/problem.hh

exercises/exercise-runtimeparams/problem.hh

...ises/solution/exercise-biomineralization/biominproblem.hh

...n/exercise-coupling-ff-pm/interface/freeflowsubproblem.hh

...ercise-coupling-ff-pm/interface/porousmediumsubproblem.hh

...tion/exercise-coupling-ff-pm/models/freeflowsubproblem.hh

.../exercise-coupling-ff-pm/models/porousmediumsubproblem.hh

.../exercise-coupling-ff-pm/turbulence/freeflowsubproblem.hh

...rcise-coupling-ff-pm/turbulence/porousmediumsubproblem.hh

exercises/solution/exercise-fluidsystem/2p2cproblem.hh