Remove old msc ver. Freecad 0.20 require 64 bit and windows 7 or higher. For this reason, msc libraries less than 1800 will no longer be used. (#7036)

* remove msc<1800

src/Base/Builder3D.cpp

@@ -172,10 +172,7 @@ void Builder3D::addText(const Base::Vector3f &vec,const char * text, float color
 
 void Builder3D::clear ()
 {
-  // Under VC6 string::clear() doesn't exist, under gcc stringstream::str() returns a copy not a reference
-#if defined(_MSC_VER) && _MSC_VER >= 1400
   result.str().clear();
-#endif
   result.clear();
 }