MatrixFree: fix partition of ghost faces for FE_Nothing

[peterrum]

As pointed out in #14342, there is/was an issue in assigning ghost faces to the right partition in the case that FE_Nothing is used, with the result that overlapping of communication/computation did not work properly. For the time being, we have disabled in #14553 the overlap if FE_Nothing is detected. However, in #14342 we have discussed better alternatives.