Removed unused example applications.

Actually, the program spu_2p is the only place the class GravityColumnSolver
is currently used, and it is also the only program capable of doing an operator
splitting with gravity segregation being solved by full Newton-Raphson (not by
columns). These features should be made available by refactoring the transport
solvers: making the segregation solvers inherit TransportSolverTwophaseInterface
and creating a general composite solver.