Move `removePlayer()` and `unsetPlayer()` to `wp.media.mixin` so that other classes can take advantage of them. 

* `removePlayer()` is an alternative to MediaElement's player removal method that does not re-add the audio|video element back to the DOM.
* `unsetPlayer()` will check for an existing `player` property on the passed instance and pause all players before calling `unsetPlayer()`

See #27389.



git-svn-id: https://develop.svn.wordpress.org/trunk@27538 602fd350-edb4-49c9-b593-d223f7449a82