Clear value for obsolete options field

The operation field isn't used for anything in our documentation. This was introduced in d5c501a2 and it looks like we missed cleaning it up in later commits that changed every other operation field to null.

docs/includes/apiargs-common-option.yaml

@@ -33,7 +33,7 @@ description: |
   The default read concern to use for {{resource}} operations. Defaults to the
   {{parent}}'s read concern.
 interface: phpmethod
-operation: selectCollection
+operation: ~
 optional: true
 replacement:
   resource: "collection"