Fix /sync timeline state duplicated in state when lazy-loading

[MadLittleMods]

Fix /sync timeline state duplicated in state when lazy-loading

Noticed while working on #19463

Related to #18793 as "[...] I wouldn't expect the leave event to be in the state block because in non-msc4222 mode, state events in the timeline block are added to state and don't need to be duplicated, so that's fun. It's not really what this bug is about, but included for context nonetheless."

Dev notes

Looks like this regressed in matrix-org/synapse#13477 (2022-08-18)

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