data store: remove redundant safety check (#1631)

* This was checking whether a node already exists in the tree before
  creating a new one.
* However, the `$index` already serves this function (nodes will be
  retrieved from the index if they already exist) so this check was
  redundant.
* This reduces the cost of the `createTreeNode` routine when the
  number of sibling elements is large.

Author: oliver-sanders

src/store/workflows.module.js

@@ -300,12 +300,7 @@ function update (state, updatedData) {
 
   // create a new tree item
   let treeParent
-  const ret = createTreeNode(state, id, tokens, updatedData)
-  if (!ret) {
-    // node already exists, nothing more to do here
-    return
-  }
-  [treeParent, treeItem] = ret
+  [treeParent, treeItem] = createTreeNode(state, id, tokens, updatedData)
 
   // add the new item to the tree
   addChild(treeParent, treeItem)
@@ -418,11 +413,6 @@ function createTreeNode (state, id, tokens, node) {
     }
   }
 
-  if (pointer.children.some(child => child.id === id)) {
-    // node already in the tree
-    return
-  }
-
   const treeNode = {
     id,
     tokens,