Remove unneeded term, fixing oil injecting behaviour.

The 'comp_term' is supposed to be a total divergence term, which is supposed
to be zero for incompressible flow. It was added for improved robustness in
stagnant areas, but as implemented it would not be computed properly for
oil injection scenarios, due to the convention for two-phase transport
source terms (positive terms are inflow of first phase [water], negative
terms are total outflow).
Merge pull request #401 from atgeirr/fix-oilinjection

Fix oil injecting behaviour of TransportSolverTwophaseReorder.