Setting the `timescaledb.restoring` guc explicitly to 'off'
for the db meant that the setting got exported in `pg_dumpall`
and some other cases where that setting would then conflict
with the setting set by the pre_restore function causing it to
be overridden and causing errors on restore. This changes to
`RESET` so that instead it will take the system default and not
be dumped separately as an override.