Trim the list of people of use minotar.net

[LukeHandle]

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...)
• www.enjin.com - cravatar.eu
• www.buycraft.net - cravatar.eu
• webcraftdev.com - dead?
• www.planetminecraft.com - internal I believe
• minecraft-skins.com - dead?
• www.massivecraft.com - dynmap generated