Delete wmail-prerelease (#3610)

Casks/wmail-prerelease.rb

@@ -1,15 +0,0 @@
-cask 'wmail-prerelease' do
-  version '2.3.1'
-  sha256 '06195daef38df002c26058ace0efbc8d86e8778150c9be27d48d0aaec7e68328'
-
-  # github.com/Thomas101/wmail was verified as official when first introduced to the cask
-  url "https://github.com/Thomas101/wmail/releases/download/v#{version}/WMail_#{version.dots_to_underscores}_prerelease_osx.dmg"
-  appcast 'https://github.com/Thomas101/wmail/releases.atom',
-          checkpoint: '8bdfc9efd95dffeb007621e2c8d8959f1f8eb490ff72694508bab598e8cbb1e5'
-  name 'WMail'
-  homepage 'https://thomas101.github.io/wmail/'
-
-  conflicts_with cask: 'wmail'
-
-  app 'WMail.app'
-end