Remove barback from the repo.

pkg/barback/example/lazy_transformer/README.md

-This example shows how to write a lazy transformer.
-
-This lazy tranformer implements a ROT13 converter. ROT13 (ROTate 13)
-is a classic substitution cipher which replaces each letter in the source
-file with the corresponding letter 13 places later in the alphabet.
-The source file should have a ".txt" extension and the converted file
-is created with a ".shhhh" extension.
-
-Generally, only transformers that take a long time to run should be made lazy.
-This transformer is not particularly slow, but imagine that it might be used
-to convert the entire library of congress–laziness would then be a virtue.
-
-For more information, see Writing a Lazy Transformer at:
-https://www.dartlang.org/tools/pub/transformers/lazy-transformer.html