registry: remove unneeded time.Duration()

These constants don't need to use time.Duration(). Fixup this file since
it seems to be the only one using this style.

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
Merge pull request #6277 from philips/remove-unneeded-time-duration

registry: remove unneeded time.Duration()