Remove divergence: do alert about absent subjects in any "non-study" dataset

Well, any BIDS dataset is a "study" dataset, but there the point is that ATM for both
"raw" and "derivative" types we expect to have sub- folders and that was the prior
behavior, which should not be affected by this PR.

This should address the review comment of @effigies
https://github.com/bids-standard/bids-specification/pull/1972/files#r2142639988

Author: yarikoptic

src/schema/rules/checks/dataset.yaml

@@ -6,11 +6,11 @@ SubjectFolders:
   issue:
     code: SUBJECT_FOLDERS
     message: |
-      There are no subject directories (labeled "sub-*") in the root of this raw BIDS dataset.
+      There are no subject directories (labeled "sub-*") in the root of this BIDS dataset.
     level: warning
   selectors:
     - path == '/dataset_description.json'
-    - dataset.dataset_description.DatasetType == "raw"
+    - dataset.dataset_description.DatasetType != "study"
   checks:
     - length(dataset.subjects.sub_dirs) > 0