Remove old GCC<=8 (#20508)

GCC version is always greater than 8

src/Mod/Sketcher/App/ExternalGeometryExtension.cpp

@@ -82,11 +82,7 @@ std::unique_ptr<Part::GeometryExtension> ExternalGeometryExtension::copy() const
 
     copyAttributes(cpy.get());
 
-#if defined(__GNUC__) && (__GNUC__ <= 4)
-    return std::move(cpy);
-#else
     return cpy;
-#endif
 }
 
 PyObject* ExternalGeometryExtension::getPyObject()