propose remove "Design" variable

[mwvoss]

I propose we remove the "Design" variable and only code studies that are Intervention designs within this sheet:
https://docs.google.com/spreadsheets/d/1nWcS0UrfvnPupAwb4uvaMeY7CqmFqIu09F3I9k4ND3s/edit?usp=sharing

This would result in:
-changing the corresponding entry in the CodeBook, categorical_key_values.json, removing study entries that have NA or Acute for their Design value, and removing the column for the Design variable in the main data sheet.