[cleanup] remove fallback implementation of decltype

Now that we require a C++11-compliant compiler, we can remove the
fallback implementation DUNE_DECLTYPE and replace it with decltype.

Note that with this change, you will also need to update PDELab to at
least 8c3e2598150d801 (master) / 485b0e3cb5897c604607 (releases/2.4)
to avoid compilation failures.

See merge request !3