Remove polyfills

[bdougherty]

We include Promise and WeakMap polyfills to the library, but both have been supported for ~10 years now. Removing both means that we can reduce the bundle size and remove all external dependencies for the production build.

Will need to think a little bit about possible implications of this changing, and of course it would need to be a major version.

[jybleau]

That would essentially drop support for IE 11: a good thing.
And it seems IE 11 only needs Promise, since it supports WeakMap

BTW, this part of the code is for IE8-9 and can probably be pruned since the player does not support IE<11.