Change: include fmt.h C++ headers in stdafx.h

This to prevent compilation issues between runs with and without precompiled
headers. Also remove the headers from the rest of the code base as they are
not needed there anymore, although they do relatively little harm.

src/group.h

@@ -16,7 +16,6 @@
 #include "vehicle_type.h"
 #include "engine_type.h"
 #include "livery.h"
-#include <string>
 
 typedef Pool<Group, GroupID, 16, 64000> GroupPool;
 extern GroupPool _group_pool; ///< Pool of groups.