Remove the UseCounters for the capture attribute

Both of the counters are at 0%:
http://www.chromestatus.com/metrics/feature/timeline/popularity/86
http://www.chromestatus.com/metrics/feature/timeline/popularity/245

In other worlds, there's hardly anyone to see the deprecation message.

BUG=240252

Review URL: https://codereview.chromium.org/303473004

git-svn-id: svn://svn.chromium.org/blink/trunk@174966 bbb929c8-8fbe-4397-9dbb-9b2b20218538