Blake Erickson 7360a0f70f DEV: Drop old IE11 intersection-observer references
It's been awhile since we have supported IE11 so it should be safe to remove
IntersectionObserver now.

From a TODO task in this repo:
> drop when we eventually drop IE11

Announcement of when we removed IE11 support:

https://meta.discourse.org/t/137984/40?u=blake
2021-05-04 10:02:03 -06:00
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