Remove flawed MSC4311 partial implementation: Client-side API's like /sync should still use stripped events

[MadLittleMods]

Remove flawed MSC4311 partial implementation: Client side API's like /sync should still use stripped events.

This was originally introduced in 0eb7252 (no PR I assume because part of Hydra security fix)

Spawning from reviewing #19722 and noticing that we have TestMSC4311FullCreateEventOnStrippedState in Complement which already passes even though that test looks flawed:

I think this test is mixing up what MSC4311 proposes. Perhaps these were changes to the MSC that came after?

For the client API's like /sync, it only proposes that m.room.create is a required stripped state event.

For the federation API's, alongside requiring m.room.create, it also mandates using the full event PDU format for all events in the invite_room_state/knock_room_state on m.room.member events (in unsigned)

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Part of #19414

Todo

• Fix/remove TestMSC4311FullCreateEventOnStrippedState Complement test so it doesn't look for the full PDU in /sync.
  • It looks like Try to modify MSC4311 test to meet new proposal matrix-org/complement#796 tries to address this

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