Event format example has extra field `origin` in it

[turt2live]

Link to problem area: https://spec.matrix.org/v1.6/rooms/v10/#event-format-1

Issue
The format schema no longer includes origin, but it's still in the example.

Expected behaviour
Delete the origin from the example.

[turt2live]

This is rooted from #998

Note that v1 (and possibly other room versions) might still intend to actually use origin, making that PR incorrect in removing the field from everywhere - this needs validation.

[richvdh]

Note that v1 (and possibly other room versions) might still intend to actually use origin, making that PR incorrect in removing the field from everywhere - this needs validation.

I'm as sure as is plausible that nothing uses origin, which was the justification for #998, and origin's subsequent removal in ruma and dendrite. It's also tested via sytest.