_brew_cask completion re-license to BSD

The original script was MIT licensed, removing this license and
relicensing it under Homebrew's default license (BSD 2 Clause).

Patrick Stadler (https://github.com/pstadler) gave his ok on this at:
https://github.com/joshka/brew-cask-zsh-completions/issues/1

share/zsh/site-functions/_brew_cask

@@ -4,29 +4,9 @@
 # Zsh Autocompletion script for Homebrew Cask
 #   https://github.com/homebrew/brew
 
-# The MIT License (MIT)
+# Authors:
-#
+#   Patrick Stadler (https://github.com/pstadler)
-# Copyright (c) 2009-2016 Robby Russell and contributors
+#   Josh McKinney (https://github.com/joshka)
-# See the full list at https://github.com/robbyrussell/oh-my-zsh/contributors
-# Copyright (c) 2016 Joshua McKinney
-#
-# Permission is hereby granted, free of charge, to any person obtaining a copy
-# of this software and associated documentation files (the "Software"), to deal
-# in the Software without restriction, including without limitation the rights
-# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
-# copies of the Software, and to permit persons to whom the Software is
-# furnished to do so, subject to the following conditions:
-#
-# The above copyright notice and this permission notice shall be included in all
-# copies or substantial portions of the Software.
-#
-# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
-# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
-# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
-# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
-# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
-# SOFTWARE.
 
 # only display the main commands (but enable completing aliases like 'ls')
 zstyle -T ':completion:*:*:*:brew-cask:*' tag-order && \