Merge branch 'js/no-more-legacy-stash'

Removal of unused code and doc.

* js/no-more-legacy-stash:
  stash: stop warning about the obsolete `stash.useBuiltin` config setting
  stash: remove documentation for `stash.useBuiltin`
  add: remove support for `git-legacy-stash`
  git-sh-setup: remove remnant bits referring to `git-legacy-stash`
