Get rid of fragile, undocumented way to get avatar url

[dartrax]

@mundschenk-at wrote:

The assumptions regarding hash generation will probably not hold forever and so the current implementation is very, very fragile. In fact, it will probably not work for some people today because Avatar Privacy up to and including 2.3.x encodes the “object type” (anonymous comment or registered user) into the subdirectory, without even considering local avatars, default avatars etc.
The subdirectory is generated here, though I’ve removed that logic from (the currently unreleased) version 2.4.0.

More on that here