Update properties element in PostHogEvent

[davemurphysf]

As part of the move to composeWebhook, plugins will be using events of type PostHogEvent instead of ProcessedPluginEvent. PostHogEvents are created here from RawClickHouseEvent (definition) and will always have $elements_chain set on its properties object (potentially with other elements, but possibly not).

[tiina303]

Let's not do this. This is user facing interface and elements_chain is specific to only posthog-js autocapture events. Internally yes we currently have elements chain there, but we should be free to change this in the future to remove it.

See related discussion #56 (comment)

Elements chain is not special if we want to change this we should do so for all the other properties too:

* $ip - for ip address, which will be removed later if project settings anonymize_ips is true
     * $set - for person properties to set
     * $set_once - for person properties to set if not already set
     * $elements_chain - for autocapture elements chain

^ again note that not all events will contain these & originally the user sent the event to us having these within properties, not separately in any way, so that matches what our SDKs do more if we leave it within properties.