Partially revert 1741ca28 (for SolverError and NumProc).

These classes might need some more revision before being approved
to be moved to the new namespace Dune::Solvers.

dune/solvers/common/numproc.hh

@@ -5,9 +5,6 @@
 
 #include <dune/common/exceptions.hh>
 
-namespace Dune {
-namespace Solvers {
-
 /** \brief Exception thrown by solvers */
 class SolverError : public Dune::Exception {};
 
@@ -51,11 +48,5 @@ class SolverError : public Dune::Exception {};
         return lhs;
     }
 
-} /* namespace Solvers */
-} /* namespace Dune */
-
-// For backward compatibility: will be removed eventually
-using Dune::Solvers::SolverError;
-using Dune::Solvers::NumProc;
 
 #endif