Reverts element-hq/synapse#18260
It is causing a failure when building release debs for `debian:bullseye`
with the following error:
```
sqlite3.OperationalError: near "RETURNING": syntax error
```
R30v2 has been out since 2021-07-19 (https://github.com/matrix-org/synapse/pull/10332)
and we started collecting stats on 2021-08-16. Since it's been over a year now
(almost 2 years), this is enough grace period for us to now rip it out.
