We log to journald anyway. There's no need for double-logging.

It should not that matrix-synapse logs to journald and to files,
but that's likely to change in the future as well.
Because Synapse's logs are insanely verbose right now (and may get
dropped by journald), it's more reliable to have file-logging too.

As Synapse matures and gets more stable, logging should hopefully
get less, we should be able to only use journald and stop writing to
files for it as well.