Blake Erickson 1b02dce594
DEV: Drop old IE11 intersection-observer references (#12942)
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 17:54:54 -06:00
..
2021-03-02 17:11:14 +00:00
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