Remove barback from the repo.

pkg/barback/example/simple_transformer/pubspec.yaml

-name: simple_transformer
-description: This hello world example implements a simple
-             transformer that inserts a copyright string into
-             an input asset - a file that ends with ".txt".
-
-dependencies:
-  barback: ">=0.14.1 <0.16.0"
-
-# Override the barback dependency so this example always uses the version
-# of barback it's bundled with.
-dependency_overrides:
-  barback: {path: ../..}
-
-transformers:
-- simple_transformer