add description and remove unused code

conf/evolutions/default/94.sql

@@ -1,10 +1,8 @@
 # --- !Ups
+-- Remove the constraint for a distinct virtual challenge (id,name) tuple, and allow users to create virtual challenges with the same name.
 ALTER TABLE virtual_challenges DROP CONSTRAINT IF EXISTS CON_VIRTUAL_CHALLENGES_USER_ID_NAME;
 DROP INDEX IF EXISTS idx_challenges_parent_id_name;
 
 # -- !Downs
 ALTER TABLE virtual_challenges ADD CONSTRAINT CON_VIRTUAL_CHALLENGES_USER_ID_NAME
   UNIQUE (owner_id, name);
-
-ALTER TABLE challenges 
-ALTER COLUMN parent_id DROP NOT NULL;