Reduce any possible duplication in logic in `create_new_client_event`

[H-Shay]

As was noted in the discussion of #14575, it appears that some of the logic added to support the batch creation of events when creating a room could potentially be overlapping/redundant of the logic already in place to support the /batch_send endpoint. Investigate whether this is so, and reduce any duplicate logic if that is the case.

[H-Shay]

I think this has been fixed by #14675