[Cpp] Duplicate `using namespace` lines.

```cpp

// Generated from WhixyLexer.g4 by ANTLR 4.13.2

#include "WhixyLexer.h"

using namespace antlr4;

using namespace Whixy;

using namespace antlr4;
```

This is a tiny tiny issue but the C++ generator is creating duplicate 'using namespace' lines.