event_forward_extremities are not correctly cleared by persistence of successor events

[matrixbot]

This issue has been migrated from #8913.

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Suppose:

• we have a linear event DAG, A <- B <- ... <- C, where C is a regular event, which references B in its state.
• our homeserver has persisted up to event A, hence has a single event_forward_extremity of A.
• our homeserver goes offline for a bit, so misses out on B and immediate successors
• our homeserver receives C over federation, hence pulls in B to complete the state at C

Empirically:

• we have all the state we need for B: it is not marked as an outlier
• we end up with event_forward_extremities of A and C

We have all the successors for A we should ever expect to receive, so I assert that it should no longer be considered an extremity. In particular, there is no way it will ever cease to be a forward extremity, so it contributes to the accumulation of forward extremities (cf #1760).