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.

Ran js-tests.
Checked that high-DPI images still appeared in places where they were

  expected.