Remove some old forge hacks

Forge apparently removed all `.pack.xz` files without warning.
It broke a bunch of stuff, as always. But it also means we don't need some ugly code anymore.

This is removed:

- Support for 'forge-pack-xz' and the forge-specific file download compression.
- The pack200 library we no longer need.

This stays:

- The LZMA decompression library - we may still want to use it.

api/logic/minecraft/MinecraftUpdate.cpp

@@ -14,7 +14,6 @@
  */
 
 #include "Env.h"
-#include <minecraft/forge/ForgeXzDownload.h>
 #include "MinecraftUpdate.h"
 #include "MinecraftInstance.h"