Remove redundant attribute-to-event mappings in HTMLMediaElement

See bug for why beforeload was left in.

BUG=308047

Review URL: https://codereview.chromium.org/27154004

git-svn-id: svn://svn.chromium.org/blink/trunk@159813 bbb929c8-8fbe-4397-9dbb-9b2b20218538