fix multiple calls to the options method when using the ajax-support mixin

[kketch]

The ajax-support mixin currently calls ember-data adapter's ajaxOptions method which is overriden by the same mixin and calls the options method. But the options method is then again called by the ajaxService, as a result it is called twice.

To fix this ajaxOptions is not overriden anymore in the ajax-support mixin, and is
not called anymore by the ajax method, as it should not be needed by ember-ajax I think
and is a private method of the ember-data adapter.

[alexlafroscia]

Hey, thanks for submitting this. I think this makes a lot of sense, since ajaxOptions doesn't actually get called anywhere other than the ajax method we're overwriting anyway!