Can't delete project libraries

I want to delete Tomahawk libraries from one of my Eclipse projects, but I get this error:

Is this some kind of library pollution? Why cant I delete it? I did remove all xmlns calls I had on my pages and also remo开发者_Go百科ved calls to all classes from that .jar file.

Try this http://dev.eclipse.org/newslists/news.eclipse.webtools/msg13723.html

or else remove that jar navigating through explorer