Remove unnecessary JoinOverTimeRange step from dataflow plan

[courtneyholcomb]

Description

Currently, for all cumulative metrics, we add a JoinOverTimeRange step to the dataflow plan. However, some cumulative metrics are able to be queried without metric_time, in which case we should not JoinOverTimeRange. When we visit that node unnecessarily, there are two repercussions:

1. We skip a SQL table alias. No major consequence here, but it is confusing to look at the SQL and wonder why it's skipped.
2. We assume inaccurate costing, since the JoinOverTimeRangeNode assumes num_aggregations=1. This results in num_aggregations being off by 1 for the overall query.

This PR removes that unneeded step from the dataflow plan.

[github-actions]

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[tlento]

Thanks for splitting this out into such an easily readable PR. The previous silliness is so obvious here!