Remove HTMLMediaElement.startTime

It was replaced with initialTime in August 2010:
http://html5.org/r/5310

BUG=309473

Review URL: https://codereview.chromium.org/32583003

git-svn-id: svn://svn.chromium.org/blink/trunk@160174 bbb929c8-8fbe-4397-9dbb-9b2b20218538