Fixed user avatars set in avatar field not being used by get_avatar.

Pods was trying to use `$avatar_field . ".ID"` as meta key to retrieve the user's custom avatar from `get_user_meta`. However, `get_user_meta` does not interpret the dot as a property accessor but will simply try to look for a meta entry with that complete key. Of course, since there's no actual record named `avatar.ID` in the user meta table, it wouldn't find anything and revert to the default avatar.

By removing the `.ID` suffix, WordPress will correctly retrieve the meta record and deserialize its value as an object. From that point on, `pods_image_id_from_field` finds the `ID` property in the passed object so `pods_image` can produce the custom avatar's HTML. Basically, all the pieces were already in place, but they didn't quite fit yet.