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.

CMakeLists.txt

@@ -248,7 +248,6 @@ add_subdirectory(libraries/launcher) # java based launcher part for Minecraft
 add_subdirectory(libraries/javacheck) # java compatibility checker
 add_subdirectory(libraries/xz-embedded) # xz compression
 add_subdirectory(libraries/quazip) # zip manipulation library
-add_subdirectory(libraries/pack200) # java pack200 compression
 add_subdirectory(libraries/rainbow) # Qt extension for colors
 add_subdirectory(libraries/iconfix) # fork of Qt's QIcon loader
 add_subdirectory(libraries/LocalPeer) # fork of a library from Qt solutions