Remove nested retries on checkout. Since the outer loop handles retries
specially, we want to let it exclusively handle retries. This will
prevent the inner retry loop triggering a timeout condition (rather than
a failure condition) with the outer loop.

BUG=chromium:665116
TEST=None

Change-Id: I4bf0c08b75861bf1c95060452118c314a121f9f3
Reviewed-on: https://chromium-review.googlesource.com/411397Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Commit-Queue: Ryan Tseng <hinoka@chromium.org>