[doc] Remove unused variable from sample code

Removes lexer since it's not used.

docs/en/reference/dql-doctrine-query-language.rst

@@ -719,8 +719,6 @@ classes have to implement the base class :
 
         public function parse(\Doctrine\ORM\Query\Parser $parser)
         {
-            $lexer = $parser->getLexer();
-
             $parser->match(Lexer::T_IDENTIFIER);
             $parser->match(Lexer::T_OPEN_PARENTHESIS);