It's not needed anymore, and we should remove it while we're in the area. Includes other misc changes to the schema layout.
This commit clarifies the required keys for each type of event that appears in sync, fixes the core event schema not declaring 'content' as required, and includes a mention that events may not have a room_id when appearing in /sync. Fixes https://github.com/matrix-org/matrix-doc/issues/595 Fixes https://github.com/matrix-org/matrix-doc/issues/909