Remove barback from the repo.

pkg/barback/example/markdown_converter/pubspec.yaml

-name: markdown_converter
-description: This hello world example implements a simple
-             transformer that converts a markdown file (with
-             a ".mdown", ".md", or a ".markdown" extension) to HTML.
-
-dependencies:
-  barback: ">=0.14.1 <0.16.0"
-  markdown: any
-
-# Override the barback dependency so this example always uses the version
-# of barback it's bundled with.
-dependency_overrides:
-  barback: {path: ../..}
-
-transformers:
-- markdown_converter