Remove pkg/ioutils's cancelReadCloser type and the vendored copy of
golang.org/x/net/context which it needed, neither of which was used
anywhere else in here. This should let us sidestep any potential
incompatibilities with the standard library's context package, which was
introduced in golang 1.7.
Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>