Remove the cmake variable DUNE_DEFAULT_LIBS

Summary

The variable DUNE_DEFAULT_LIBS is exactly identical to DUNE_LIBS and can thus be removed.

cmake/modules/DuneMacros.cmake

@@ -618,7 +618,6 @@ macro(dune_process_dependency_macros)
       if(${_mod}_LIBRARIES)
         message(STATUS "Setting ${_mod}_LIBRARIES=${${_mod}_LIBRARIES}")
         foreach(_lib ${${_mod}_LIBRARIES})
-          list(INSERT DUNE_DEFAULT_LIBS 0 "${_lib}")
           list(INSERT DUNE_LIBS 0 "${_lib}")
         endforeach()
       endif()
@@ -953,7 +952,7 @@ endif()
 endmacro(finalize_dune_project)
 
 macro(target_link_dune_default_libraries _target)
-  foreach(_lib ${DUNE_DEFAULT_LIBS})
+  foreach(_lib ${DUNE_LIBS})
     target_link_libraries(${_target} PUBLIC ${_lib})
   endforeach()
 endmacro(target_link_dune_default_libraries)