feat: improve enrollment query performance by removing CTEs

[SoryRawyer]

Following advice from Altinity, this PR removes CTEs from the fact_enrollments and fact_enrollments_by_day dataset queries. The Clickhouse benchmark tooling shows speedups and the query plans for these queries are shorter than before.

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[SoryRawyer]

Ah, after doing some more load testing, this seems to perform worse than the current version, so I'll close this.

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