Retire support for `sha1` stanza.

We haven't received new submissions using `sha1` for quite
some time.

**HOLD** this until pending relevant PRs in caskroom/homebrew-versions are merged:
 - caskroom/homebrew-versions#256
 - caskroom/homebrew-versions#257
 - caskroom/homebrew-versions#258
 - caskroom/homebrew-versions#259
 - caskroom/homebrew-versions#260
 - caskroom/homebrew-versions#261
 - caskroom/homebrew-versions#262

`sha1` is already removed from all Casks in homebrew-unofficial, homebrew-fonts, and the main repo.