do not cascade with removal of base_study (#620)

Author: jdkent

store/neurostore/models/data.py

@@ -150,8 +150,7 @@ class BaseStudy(BaseMixin, db.Model):
     user = relationship("User", backref=backref("base_studies"))
     # retrieve versions of same study
     versions = relationship(
-        "Study", backref=backref("base_study"), cascade="all, delete-orphan"
-    )
+        "Study", backref=backref("base_study"))
 
     def update_has_images_and_points(self):
         # Calculate has_images and has_coordinates for the BaseStudy