[cleanup][sequential] Remove old sequential headers that made it past the last delete

5 jobs for !2787 with cleanup/remove-sequential
latest detached
Status Name Job ID Coverage
  Trigger
manual full-dune-2.7-clang
manual full-dune-2.7-gcc
manual full-dune-master-gcc
manual minimal-dune-2.7-gcc
 
  Downstream Modules
created trigger lecture