Simplify components with unnecessary browser polyfills/fallbacks/hacks

[thompsongl]

Browsers supported by EUI has changed in the last year, most notably the removal of IE11 from the list. As such, several components have residual code to support one or more browsers that no longer require support and could be improved or simplified. This includes areas such as web APIs, browser eventing, CSS features, etc.

Use this issue to track potential refactors.

• EuiResizeObserver. The ResizeObserver API is now supported by all supported browsers. The makeCompatibleObserver fallback method is no longer necessary.

[cchaos]

I'd like to give this issue a bump, specifically in regards to the EuiResizeObserver. I think it's starting to get used a lot more and I even have plans to use it in a lot more components. But recently I've noticed some people mentioning performance/issues with using a lot of this component. For example this issue in Kibana: elastic/kibana#22913 (comment)

[thompsongl]

The makeCompatibleObserver fallback method is no longer necessary.

This is true, except for jest environments. jsdom does not implement the ResizeObserver API, which means that jest is using the MutationObserver fallback in makeCompatibleObserver. It appears that resize-observer-polyfill often recommended for jest environments uses a similar MutationObserver fallback approach.
So, by removing our MutationObserver fallback and relying solely on ResizeObserver, we'll lose the ability to test EuiResizeObserver and useResizeObserver. This seems fine, because our supported browsers all implement the ResizeObserver API, which means our tests were not actually testing the real, intended behavior of target browsers.

[daveyholler]

Closing as done. IE stuff has been purged and the polyfill has been removed. @constancecchen will update with links

[cee-chen]

Looks like Greg already updated EuiResizeObserver in #4709, and IE polyfills were removed in #6162