Remove legacy event_names code

[timgl]

Changes

We're now exclusively using EventDefinition and PropertyDefinition. I want to make some changes to that but for #4166 but I want to refactor this first.

Note: this also removes volume stats for individual properties. I don't think they're as useful as events, and those queries are super expensive. It'll also allow me to do the next step, which will be to link properties to individual events, at which point you probably only need the event stats instead of both.

Checklist

• All querysets/queries filter by Organization, by Team, and by User
• Django backend tests
• Jest frontend tests
• Cypress end-to-end tests
• Migrations are safe to run at scale (e.g. PostHog Cloud) – present proof if not obvious

[paolodamico]

lgtm! made a few tweaks and fixed tests