Remove methods that depend on EventContent::from_parts

[zecakeh]

Even if we can't remove from_parts, they can depend on the type implementing Deserialize instead.

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Preview Removed

[jplatte]

If we are keeping from_parts, maybe keep deserialize_content and adjust the docs to say "Use Raw::deserialize_as or Raw::deserialize_content"? Maybe we should also add deserialize_content_as for that one event with a variable event type (IIRC it's in account data).

[zecakeh]

Ah right we still need deserialize_content or deserialize_content_as for that one event type.

[zecakeh]

I'm actually gonna close this. If we remove StateUnsignedFromParts, then we have to find another way for the fallback CustomStateEventContent type to get the event_type.