revert(conversations): un-revert unread count feature flag removal

This reverts commit 87df43bdd0 from #14623, reapplying the conversation unread count feature flag removal from #14610.

app/javascript/dashboard/store/modules/conversationUnreadCounts.js

@@ -48,9 +48,6 @@ export const actions = {
       // Ignore errors so the sidebar can continue rendering without badges.
     }
   },
-  clear({ commit }) {
-    commit(types.SET_CONVERSATION_UNREAD_COUNTS, {});
-  },
 };
 
 export const mutations = {

app/javascript/dashboard/store/modules/specs/conversationUnreadCounts/actions.spec.js

@@ -39,15 +39,4 @@ describe('#actions', () => {
       expect(commit).not.toHaveBeenCalled();
     });
   });
-
-  describe('#clear', () => {
-    it('clears unread counts', () => {
-      actions.clear({ commit });
-
-      expect(commit).toHaveBeenCalledWith(
-        types.SET_CONVERSATION_UNREAD_COUNTS,
-        {}
-      );
-    });
-  });
 });