Delete rust-nightly.rb

This is [in the main homebrew-cask project now](https://github.com/caskroom/homebrew-cask/blob/master/Casks/rust.rb), so I think it shouldn't be here anymore.

Casks/rust-nightly.rb

@@ -1,12 +0,0 @@
-class RustNightly < Cask
-  arch = `uname -m`.chomp
-  arch = 'i686' if arch.empty?
-  pkgname = "rust-nightly-#{arch}-apple-darwin.pkg"
-
-  url "http://static.rust-lang.org/dist/#{pkgname}"
-  homepage 'http://www.rust-lang.org/'
-  version 'latest'
-  install pkgname
-  sha256 :no_check
-  uninstall :pkgutil => 'org.rust-lang.rust'
-end