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.

This cuts time per step with about 20 seconds (!) for me on the Norne test case.