DRY in gravatar.el

DRY in gravatar.el

For discussion, see the following thread:
https://lists.gnu.org/archive/html/emacs-devel/2019-07/msg00528.html
* lisp/image/gravatar.el (gravatar-data->image): Remove.
(gravatar-retrieve, gravatar-retrieve-synchronously): Reuse
url-fetch-from-cache and gravatar-retrieved to reduce duplication.
(gravatar-retrieved): Only cache buffer if url-current-object is
non-nil and return result of callback.  This affords reusing this
function in cached URL buffers.