The original purpose of this was to cancel downloads if pullV2Tag
returns an error, preventing an associated crash (see #15353). The
broadcaster now accomplishes the same thing that the pipe does, making
the pipe redundant. When pullV2Tag returns, all broadcasters are closed,
which means all further writes to those broadcasters will return errors.
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>