libstdc++: Remove unused header

This header was not part of the patch submitted and reviewed, and was
not added to include/Makefile.am so never gets installed anyway. It
appears to have been committed by mistake, so let's remove it.

libstdc++-v3/ChangeLog:

	* include/precompiled/expc++.h: Removed.

libstdc++-v3/include/precompiled/expc++.h

@@ -1,57 +0,0 @@
-// C++ includes used for precompiling extensions -*- C++ -*-
-
-// Copyright (C) 2006-2020 Free Software Foundation, Inc.
-//
-// This file is part of the GNU ISO C++ Library.  This library is free
-// software; you can redistribute it and/or modify it under the
-// terms of the GNU General Public License as published by the
-// Free Software Foundation; either version 3, or (at your option)
-// any later version.
-
-// This library is distributed in the hope that it will be useful,
-// but WITHOUT ANY WARRANTY; without even the implied warranty of
-// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-// GNU General Public License for more details.
-
-// Under Section 7 of GPL version 3, you are granted additional
-// permissions described in the GCC Runtime Library Exception, version
-// 3.1, as published by the Free Software Foundation.
-
-// You should have received a copy of the GNU General Public License and
-// a copy of the GCC Runtime Library Exception along with this program;
-// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
-// <http://www.gnu.org/licenses/>.
-
-/** @file expc++.h
- *  This is an implementation file for a precompiled header.
- */
-
-#if __cplusplus < 201103L
-#include <bits/extc++.h>
-#endif
-
-#if __cplusplus > 201103L
-#include <experimental/algorithm>
-#include <experimental/any>
-#include <experimental/chrono>
-#include <experimental/deque>
-#include <experimental/filesystem>
-#include <experimental/forward_list>
-#include <experimental/functional>
-#include <experimental/iterator>
-#include <experimental/list>
-#include <experimental/map>
-#include <experimental/memory>
-#include <experimental/numeric>
-#include <experimental/optional>
-#include <experimental/ratio>
-#include <experimental/set>
-#include <experimental/string>
-#include <experimental/string_view>
-#include <experimental/system_error>
-#include <experimental/tuple>
-#include <experimental/type_traits>
-#include <experimental/unordered_map>
-#include <experimental/unordered_set>
-#include <experimental/vector>
-#endif