Revert "Derive field type with global helper."

This reverts commit 506d2de0

Besides the fact that the FieldTraits default specialization is questionable IMHO ¹, this requires additional specializations for several types from dune-istl, among them notably BCRSMatrix. In the current state of dune-istl (and dune-matrix-vector) this small change is not worth the effort.

1. It yields the very type if no specialization is found. This is reasonable for arithmetic types, but leads to weird behaviour for complex types that do not have a specialization. I tried to implement a specialization based on std::is_arithmetic and forwarding otherwise to T::field_type, but ended not proposing this to dune-common, in particular because I did not want to investigate what looked to me like classes explicitly exploiting the current behaviour (or maybe a hidden bug), e.g. something like AxisAligned... ↩