fix: user can't reverse a class parent decision

Class parent defaults to Root but user can't select it later. This
should be reversible, especially given how much functionality this
affects. Fixes #256

deps/db/src/logseq/db/frontend/class.cljs

@@ -110,7 +110,7 @@
 (def private-tags
   "Built-in classes that are private and should not be used by a user directly.
   These used to be in block/type"
-  (set/union internal-tags
+  (set/union (disj internal-tags :logseq.class/Root)
              #{:logseq.class/Journal :logseq.class/Whiteboard}))
 
 (def hidden-tags