We had a bunch of JS code to enable us to work with images on high-DPI screens which is no longer useful. The gravatar-related methods in here have long been unused, and the `Djblets.enableRetinaImages` method was effectively a no-op in modern browsers that support `srcset` natively. The calls to that have been removed in Review Board, and this change removes the implementation. Testing Done: - Ran js-tests. - Checked that high-DPI images still appeared in places where they were expected.