Remove unused grav_ member and function used to compute it.

This is a relic of the way we originally handled gravity. The member
remained after the change, and is now a major time-consumption sink
due to the unfortunate fact that it is computed every time step
(unnecessary), and because the gravityOperator() function (now removed)
was very unperformant after changing to use the faceCells() function.
Merge pull request #175 from atgeirr/remove-unused-grav

Remove unused grav_ member and function used to compute it.