* Clean up after V2 migration

This PR removes all the leftovers and alternative code
branching after v2 migration.

The self-hosted release is being drafted at:

https://github.com/plausible/hosting/issues/68

Refs:
  - https://github.com/plausible/analytics/pull/2865
  - https://github.com/plausible/analytics/pull/2825
  - https://github.com/plausible/analytics/pull/2780

* !fixup