Remove unused parts of MediaControllerInterface

Remove the parts of MediaControllerInterface that aren't actually used
by the MediaControls code, which is the only place where this abstract
interface is used as opposed to HTMLMediaElement or MediaController.

hasCurrentSrc() wasn't used anywhere, so remove it.

BUG=341852

Review URL: https://codereview.chromium.org/156173003

git-svn-id: svn://svn.chromium.org/blink/trunk@166766 bbb929c8-8fbe-4397-9dbb-9b2b20218538