Do not use `json` dataType param for delete requests using native DELETE method. See #2218.

It seems it is a backbone issue after all. Backbone.sync explicitly set `dataType` for jqXHR object to `json` which cause empty responses from DELETE requests made with jQuery 1.9+ to fail: http://jquery.com/upgrade-guide/1.9/#jquery-ajax-returning-a-json-result-of-an-empty-string

Not setting `dataType` for delete request when using HTTP `DELETE` method fixes the problem.