Sql optimise killer query2

In this commit I have decomposed the getListQuery() method's query in to two cause for the readability and for improve the performance. In the previous query before the change there was a join operation to get the un-published category ids. In this query the former join has been removed and the un-published category ids are being retrieved separately. And then those ids are used with the NOT IN and IN operations in order to improve the performance
Please use meaning full name of the variable. `$db1` is not allowed according to [Joomla Coding Standards](http://joomla.github.io/coding-standards/).