[cleanup] directory main not needed anymore.

dune/fem-dg/CMakeLists.txt

-dune_add_subdirs( algorithm assemble main misc models operator pass solver examples)
+dune_add_subdirs( algorithm assemble misc models operator pass solver examples)

dune/fem-dg/main/CMakeLists.txt

-dune_install( main.hh codegen.hh  codegen2.hh caching.hh caching2.hh vectorialbasefunctionset.hh default.hh main_0.cc main_1.cc  main_2.cc  main_3.cc  main_4.cc  main.cc main_pol.cc )