[facet][box] do not use modified elembctypes

This merge request removes the custom ElementBoundaryTypes defined for the box-facet-coupling model. This affected the facet tracer test, because the combination of bulk-facet boundary conditions makes no sense for box (see description in 4e4c35d6).

However, using the old combo of BCs led to significant undershoots in mole fraction. In the new setup, the undershoots reduce substantially.

This can help making progress related to !2112 (closed)