[Target: Cpp] fix Duplicate using namespace lines.

[cppenjoy]

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📛 Issue: https://github.com/antlr/antlr4/issues/4733

A simple bug in the ANTLR4 code generator for C++.

If we generate a lexical analyzer for any ANTLR4 grammar using target C++, then we can see that the ANTLR4 code generator duplicates "using namespace std;" twice, which is not logical behavior.

For example, if we have a small grammar:

grammar main;

mainRule: token+;
Token: 'token';

Now let's try to generate a parser for this grammar:

antlr4 main.g4 -Dlanguage=Cpp

Then we will get a lexical file in the likeness:

#include "MainLexer.h"

using namespace antlr4;

using namespace antlr4;