[BI-2449] - Correct Exp Preview Summary Stats

[HMS17]

Description

Story: BI-2449 - Correct Exp Preview Summary Stats

Renaming and removing some of the experimental summary preview stats.

Dependencies

bi-api: develop

Testing

• Go to experiment details
• Ensure that the "Observation Unit", "Observations with Data", and "Observations without Data" items are no longer present in the summary stats
• Ensure that the items "Observations" and "Observation Variables" are present and have the correct values

Checklist:

• I have performed a self-review of my own code
• I have tested my code and ensured it meets the acceptance criteria of the story
• I have create/modified unit tests to cover this change
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to documentation
• I have run TAF: <link to TAF run>
• I have run SiteImprove on pages impacted by changes