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

lib/tasks/javascript.rake

@@ -140,9 +140,6 @@ def dependencies
       source: 'mousetrap/plugins/global-bind/mousetrap-global-bind.js'
     }, {
       source: 'resumablejs/resumable.js'
-    }, {
-      # TODO: drop when we eventually drop IE11, this will land in iOS in version 13
-      source: 'intersection-observer/intersection-observer.js'
     }, {
       source: 'workbox-sw/build/.',
       destination: 'workbox',