Throw away everything in the go workspace except bin (2nd attempt).

Trying a more reliable way of deleting directories on Windows.
The attempt in https://codereview.chromium.org/1680173002/ doesn't
seem to help on the bots.

BUG=585462
TBR=phoglund@chromium.org

Review URL: https://codereview.chromium.org/1686753002 .
1 file changed