When editor does not respond to the request for withdrawal
what should I do? 


Send another email stating that you will consider the article retracted if you do not receive a response within one week.


  Can I submit the manuscript to another journal?


Yes, after one week.



Responses to comments:


  I would likely do this but I am not convinced that it precludes troubles. Imagine the same referee getting it twice. It would be confusing at least. Just to mention one case.


Referees regularly receive manuscripts twice, for instance, when a manuscript is rejected. Admittedly, this situation is different, since the reviewer may not have been notified of rejection. However, such a reviewer will likely assume that the paper has been rejected. Even if they don't, they will likely contact the editor, who will be able to promptly deal with the situation. This could even be pre-empted, for example, an unacknowledged retraction could be explained in a covering letter.


  [this] does not avoid...issues such as when the first journal and second journal both publish it...For all you know, the author may ALREADY have assigned copyright to the first journal.


From the question I assume copyright hasn't been assigned nor is the manuscript camera-ready.