[!764] Do not import std::shared_ptr into namespace Dune

Merge branch 'unimport-shared-ptr' into 'master'

ref:core/dune-common This is a left-over from the days where Dune had its own
implementation of std::shared_ptr. Now that that is long gone, it violates the
principle of least surprise to find shared_ptr in Dune::

See merge request [core/dune-common!764]

  [core/dune-common!764]: gitlab.dune-project.org/core/dune-common/merge_requests/764