Remove MediaControlVolumeSliderElement::m_clearMutedOnUserInteraction

It is always set to true.

Also drop the volume change check, since HTMLMediaElement::setVolume()
and MediaController::setVolume() already do that.

BUG=341813

Review URL: https://codereview.chromium.org/177423011

git-svn-id: svn://svn.chromium.org/blink/trunk@168438 bbb929c8-8fbe-4397-9dbb-9b2b20218538