Draft: Remove FieldMatrix/Vector scalar specialization

(This is an experiment whether it would be possible to remove the FieldVector<K,1> and FieldMatrix<K,1,1> specialization completely.)

See accompanying MRs in other modules:

• dune-localfunctions!284
• dune-geometry!261
• dune-istl!601
• dune-grid!767
• staging/dune-functions!502