chore: delete stale tutorial content (#331)

1. This contains an iOS app, but the main iOS tutorial linked from
https://bazel.build/start/ios-app lives elsewhere
2. It shows rules_appengine, but that repo appears to have only minimal
maintenance for broken flags. Latest comment from a maintainer is 'not
being actively maintained'
bazelbuild/rules_appengine#128 (comment)
3. bazelbuild/codelabs was archived, but there are active community
forks at https://github.com/engflow/codelabs and
https://github.com/aspect-build/codelabs which have fresher content than
this tutorials folder.
4. No one has touched tutorial/android in four years.