BUG: force TIXML_USE_STL to always true in otb_tinyxml.h

Supporting TIXML_USE_STL to false is completely useless because OTB depends on the STL. Because of !487 (merged), this causes a build error in OTB 7.0.0 when using a tinyxml not from the SuperBuild (because it was set to ON in SuperBuild/patches/TINYXML/CMakeLists.txt).

• Force TIXML_USE_STL to true in otb_tinyxml.h
• Remove support and compiler checking for setting TIXML_USE_STL from CMake.

Modules/ThirdParty/TinyXML/CMake/otbTestTinyXMLUseSTL.cxx

-/*
- * Copyright (C) 2005-2019 Centre National d'Etudes Spatiales (CNES)
- *
- * This file is part of Orfeo Toolbox
- *
- *     https://www.orfeo-toolbox.org/
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-#include <iostream>
-#include <tinyxml.h>
-
-int main(int argc, char* argv[])
-{
-  TiXmlDocument doc( NULL );
-  std::cout << doc;
-}