Remove HTMLMediaElement::pausedForUserInteraction().

It hasn't been implemented since it was added in r41907 ~5 years ago.

Review URL: https://codereview.chromium.org/314823003

git-svn-id: svn://svn.chromium.org/blink/trunk@175437 bbb929c8-8fbe-4397-9dbb-9b2b20218538