Rip out all of the beautiful bespoke Popover animation stuff

This isn't part of the landed spec [1], and will be replaced by
a combination of these five CSSWG issues:

- w3c/csswg-drafts#4441
- w3c/csswg-drafts#6429
- w3c/csswg-drafts#8174
- w3c/csswg-drafts#8189
- w3c/csswg-drafts#8389

After this CL, you will no longer be able to animate your
popover like this:

```
  [popover] {
    opacity: 0;
    transition: opacity 0.2s;
  }
  [popover]:open {
    opacity: 1;
  }
```

Instead you'll need to use CSS animations or (eventually) transitions
and you'll have to explicitly declare the `display` and `top-layer`
properties:

```
  transition: opacity 0.2s, display 0.2s, top-layer 0.2s;
```

[1] https://html.spec.whatwg.org/multipage/popover.html

Bug: 1307772,1413556
Change-Id: I4877dd69a06f2624bdb463b065b2e2b66cbf1154