COM_STMT_EXECUTE target selection is too restrictive when no metadata is provided

Whenever a COM_STMT_EXECUTE arrives with no metadata, previously it was required to be sent to the same server. With MXS-2750 fixed in 2.5, this restriction was lifted but some code was left over that still enforces the old requirement.

This can be reproduced by executing a COM_STMT_EXECUTE with parameters that is classified as a read and then putting the old target into maintenance mode. The next time the COM_STMT_EXECUTE is executed, the metadata won't be sent and MaxScale will close the connection.

The fix to this is simple, remove the code that treats a COM_STMT_EXECUTE with no metadata as a query that must be routed to the previous target.