Follow-up, replace custom EventHub implementation with mitt

Description

In this MR !35141 (merged), we introduced a custom EventHub based off of mitt with a few implementation tweaks so that it behaves like the Vue instance event hub.

Once this Epic is resolved &3334 (closed), we can start looking at removing this custom implementation for using mitt or something like that.

Full plan

• !35141 (merged) "forks mitt" so that we gain the benefit of not creating a new Vue instance for every event hub.
• #223863 (closed) Add a eslint rule to disallow calling $emit with multiple args
• &3334 (closed) Clean up our existing code
• This issue. Now we can safely adopt something like mitt