Deletes are not allowed unless they contain a "where" or "like" clause.

I see it is hard coded in the system QB, you can either implement your custom QB and override delete method or implement your own method, 
or you can add dummy where $this->db->where('1=1', NULL, FALSE);
Do little research on how to extend query builder, there are must be examples. Because it is also requires to extend the Loader library

This rule is implemented for safety reason, you may unintentionally delete all the records in the table which may cause more headache for you.

But there are two where , join but there are...