Remove fallback from get_missing_events.
Remove fallback from get_missing_events.

get_missing_events used to fallback to fetching the missing events
individually requesting from every server in the room, one by one.e

This could be unacceptably slow, possibly causing #1732

TribeMedia/synapse,TribeMedia/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,TribeMedia/synapse,TribeMedia/synapse,matrix-org/synapse,matrix-org/synapse,TribeMedia/synapse,matrix-org/synapse