Trim the list of people of use minotar.net

Used @PaulBGD's PR https://github.com/minotar/minotar.net/pull/3 as a base and trimmed it further.

 * www.minecraftforum.net - internal solution
 * bukkit.org - assumed internal as well (wasn't sure where I was looking for avatars...)
 * enjin.com - cravatar.eu
 * buycraft.net - cravatar.eu
 * webcraftdev.com - dead?
 * www.planetminecraft.com - internal I believe
 * minecraft-skins.com - dead?
 * massivecraft.com - dynmap generated