Ajax content type 11821

Seems that Django 1.6 is a bit more picky about parsing REQUEST data from jQuery AJAX calls, resulting in a couple of bugs http://trac.openmicroscopy.org.uk/ome/ticket/11819 and http://trac.openmicroscopy.org.uk/ome/ticket/11821

Solution is to remove 'contentType' from jQuery AJAX requests.

To test:
 - Add a Tag to an Image, then click the X on the tag to remove it from the Image.
 - Select several images and add to the Basket. Then go to the basket, select a couple and Remove from basket.
 - Also check that deleting a File attached to a P/D/I still works (delete not remove) - regression test.