Remove some dead stores pointed out by clang.

Change-Id: I919a2e98474fec42861391d11d2ba1225eb2adf9
Reviewed-on: http://codereview.qt.nokia.com/4083
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>
Reviewed-by: Leandro T. C. Melo <leandro.melo@nokia.com>

src/libs/3rdparty/cplusplus/Lexer.cpp

@@ -590,7 +590,6 @@ void Lexer::scan_helper(Token *tok)
                 break;
             } else if (ch == '@' && _yychar == '"') {
                 // objc @string literals
-                ch = _yychar;
                 yyinp();
                 tok->f.kind = T_AT_STRING_LITERAL;