Remove xamarin-studio-preview (#4101)

Casks/xamarin-studio-preview.rb

@@ -1,13 +0,0 @@
-cask 'xamarin-studio-preview' do
-  version '6.0.0.3668'
-  sha256 '1ed29cac15b64a6bce00f81f7898ac21c55d4c04b33372c0bf61a1d11d5ee389'
-
-  # download.xamarin.com/roslyn-preview was verified as official when first introduced to the cask
-  url "http://download.xamarin.com/roslyn-preview/XamarinStudio-#{version}.dmg"
-  appcast 'https://xampubdl.blob.core.windows.net/static/installer_assets/v3/Mac/Universal/InstallationManifest.xml',
-          checkpoint: '08fef545d50bd40a08d194b082bba6752cb27453d3381fd4c0504bdf18dd6ab2'
-  name 'Xamarin Studio Preview'
-  homepage 'https://www.visualstudio.com/vs/visual-studio-mac/'
-
-  app 'Xamarin Studio.app'
-end