Remove hover-triggered behaviors from the Pop-Up API

Rather than keeping all of this complexity in the codebase, based
on the decision [1] to "punt" this behavior to V2 of the API, I'm
going to remove it all. This CL can be reverted to bring it back.

This CL mostly reverts the CLs below, but some functionality is
left in place, such as the CSS time interpolation code. But for
the most part, this reverts these CLs:

 1. https://chromium-review.googlesource.com/c/chromium/src/+/3763422
 2. https://chromium-review.googlesource.com/c/chromium/src/+/3760885
 3. https://chromium-review.googlesource.com/c/chromium/src/+/3781406
 4. https://chromium-review.googlesource.com/c/chromium/src/+/3812260
 5. https://chromium-review.googlesource.com/c/chromium/src/+/3811136

[1] openui/open-ui#526

Bug: 1307772
Change-Id: I6c8e4c9d2cf1ef0d36df71767b26122f65ea5b5b
Fixed: 1349910
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3842343
Reviewed-by: David Baron <[email protected]>
Auto-Submit: Mason Freed <[email protected]>
Commit-Queue: Mason Freed <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1037917}