Remove Dash Beta for moving to Unofficial
Remove Dash Beta for moving to Unofficial

While making an update to `dash-beta.rb`, @vitorgalvao pointed out
that the private nature of the Dash beta meant that it belonged in
caskroom/homebrew-unofficial instead.

This commit removes the `dash-beta.rb` file pending its move.

elovelan/homebrew-versions,chadcatlett/caskroom-homebrew-versions,noamross/homebrew-versions,dictcp/homebrew-versions,zchee/homebrew-versions,zsjohny/homebrew-versions,lukaselmer/homebrew-versions,tomschlick/homebrew-versions,bimmlerd/homebrew-versions,pquentin/homebrew-versions,invl/homebrew-versions,nicday/homebrew-versions,adjohnson916/homebrew-versions,coeligena/homebrew-verscustomized,3van/homebrew-versions,visualphoenix/homebrew-versions,lukasbestle/homebrew-versions