Remove test expectations for detach-range-during-deletecontents.html.

This test was at first failing on the try bots when it was written, but it is
passing on the main bots:
http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Blink&tests=detach-range-during-deletecontents.html

It is suspected that the try bots were misbehaving for some reason.

BUG=256913
TBR=crogers@chromium.org

Review URL: https://chromiumcodereview.appspot.com/18323022

git-svn-id: svn://svn.chromium.org/blink/trunk@153553 bbb929c8-8fbe-4397-9dbb-9b2b20218538