[daggy-u] [dbt] - Correct model in Lesson 6 (DEV-180) (#20371)

## Summary & Motivation

This PR removes a problematic column in the `daily_metrics` model in
Lesson 6 of the Dagster & dbt course. It was causing issues with the
`group by` clause.

## How I Tested These Changes

dagster dev, build/run dbt, local materialization

docs/dagster-university/pages/dagster-dbt/lesson-6/2-creating-a-simple-incremental-model.md

@@ -29,7 +29,6 @@ with
         select
             date_trunc('day', pickup_datetime) as date_of_business,
             count(*) as trip_count,
-            count(*) - lag(count(*), 1) over (order by date_trunc('day', pickup_datetime)) as trip_count_change,
             sum(duration) as total_duration,
             sum(duration) / count(*) as average_duration,
             sum(total_amount) as total_amount,