emoji_settings: Stop using the avatar as placeholder; it is never seen.

The design of this element was changed midway through #22504, and
despite a separate placeholder icon using used, the preview was still
reset to the default avatar -- but is always hidden when so set.

static/js/settings_emoji.js

@@ -188,7 +188,6 @@ export function add_custom_emoji_post_render() {
         $preview_image.hide();
         $placeholder_icon.show();
         $preview_text.show();
-        $preview_image.attr("src", "/static/images/default-avatar.png");
     });
 }