Remove old emoji versions 1, 2, and 3

In 17.6 and 17.7, we upgraded our emojis support to version 4. In 17.8, we should be able to remove the old image assets that are stored in public/-/emojis/1, public/-/emojis/2, and public/-/emojis/3. We only need to keep around version 4, public/-/emojis/4.

This will reduce the number of files that we need to distribute by several thousand png files.