Fix memory leak when switching providers within a playlist [Delivers #79305128 #79031288]

[Fixes #79305128 #79031288]
I think this may be a misunderstanding due to the name **defaultProvider**. This variable is not a reference to jwplayer.html5.DefaultProvider, but it is a variable passed into the html5.model's constructor. This is useful when we want to create a model using a cached provider, as seen [here](https://github.com/jwplayer/jwplayer/blob/a7387f38e5c8418890edba0cc51ce24707afbea8/src/js/html5/jwplayer.html5.instream.js#L65)

Feel free to make a pull request with an updated comment, or even update the name of the variable if you have a better idea.