vendor/github.com/docker/docker/hack: Remove unused directory

This directory just had Markdown and vendor.conf.  I'm not sure why we
have it in our version control, maybe old versions of vndr kept it?
Or maybe folk dropped it into vendor/ by hand without using vndr?  The
history of that vendored directory is:

* 619637a (Handle Linux Capabilities from command line, 2017-11-03,
  #17) added the three files to our version control.
* c344fe6 (Update vendoring, 2017-11-22, #60) bumped hack/README.md.
* af64e10 (Vendor in lots of kubernetes stuff to shrink image size,
  2018-03-26, #554) bumped hack/README.md.
* 27107fd (Vendor in latest containers/image and contaners/storage,
  2018-04-18, #509) removed the files.
* a824186 (Use buildah commit and bud in podman, 2018-04-25, #681)
  added the files back.
* I'm removing them again in this commit.

With this commit,

  $ vndr github.com/docker/docker

becomes a no-op.

Signed-off-by: W. Trevor King <[email protected]>

Closes: #752
Approved by: baude

Author: wking