For MySQL, a query for type LEFT JOIN .. IS NULL
leads to a nested antijoin which leads to poor performance.
We fix this by rewriting the query to avoid the antijoin.
See the JIRA epic for more context behind this.
We also make another improvement to remove the DISTINCT
clause. It didn't serve any purpose since userids would
already be unique.
https://mattermost.atlassian.net/browse/MM-53406
```release-note
NONE
```
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