[periodic_gridview] remove deprecated header

dune/xt/grid/periodic_gridview.hh

-// This file is part of the dune-xt-grid project:
-//   https://github.com/dune-community/dune-xt-grid
-// Copyright 2009-2018 dune-xt-grid developers and contributors. All rights reserved.
-// License: Dual licensed as BSD 2-Clause License (http://opensource.org/licenses/BSD-2-Clause)
-//      or  GPL-2.0+ (http://opensource.org/licenses/gpl-license)
-//          with "runtime exception" (http://www.dune-project.org/license.html)
-// Authors:
-//   Felix Schindler (2015 - 2017)
-//   Rene Milk       (2015 - 2016, 2018)
-//   Tobias Leibner  (2015 - 2017)
-
-#warning This header is deprecated, include <dune/xt/grid/view/periodic.hh> instead (08.03.2017)!
-#include <dune/xt/grid/view/periodic.hh>