Fix Warning when Zephir compiles the file

Warning: Variable "queryParams" assigned but not used in Phalcon\Db\Adapter\Pdo::convertBoundParams in /usr/local/src/cphalcon/phalcon/db/adapter/pdo.zep on 433 [unused-variable]

          let queryParams = [], placeHolders = [],
        ----------------------^
Merge pull request #3075 from dugwood/patch-2

[2.0.0] Fix Warning when Zephir compiles the file