[PHP]Remove the support of named placeholders for prepare statements

PHP Driver has implemented named placeholders on the client and CUBRID engine doesn't support it. However, there will be many potential problems, for example, '?' and ':' can't be in a statement at the same time and if CUBRID engine changes it escape strings, the rule used now will not be correct.

MySQL only supports positional placeholders and PDO has implemented named and positional placeholders.

So we decide to remove the support of named placeholders.