Reverted shearing box changes to face-centered flux correction.

The recent shearing box changes broke face-centered mesh refinement. This merge request reverts the changes to flux_correct_fc.cpp to be consistent with the version from May 24. To prevent any special surprises for people using the shearing box, Mesh now contains a temporary check with a FIXME comment before it to ensure that mesh refinement and the shearing box cannot be simultaneously active.