fix remove previous backup dir in windows

Change-Id: Iaad7f7de2927cf03eadc29b4995e8e998488c19b
Reviewed-on: http://review.couchbase.org/24784
Tested-by: Thuan Nguyen <soccon@gmail.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>