title: Get error "doc.delete" is not a function when trying to delete docs in a Firestore CollectionGroup
body: <p>I have an app where a user can create a post. Users can also bookmark these posts. However, when a post author deletes his post that others may have already bookmarked, I need to also automatically delete those bookmarks that are associated to the deleted post.</p>
  <p>Here's my data model (when a user bookmarks a post, the bookmark is saved to the user's record in a subcollection called <code>bookmarks</code>:</p>
  <p><a href="https://i.stack.imgur.com/noDCE.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/noDCE.png" alt="enter image description here" /></a></p>
  <p>I've written the code logic to delete a bookmark like so:</p>
  <pre><code>          const postsQuery = await this.$fire.firestore
              .collectionGroup('bookmarks')
              .where('id', '==', this.post.id)
              .get()
            postsQuery.forEach((doc) =&gt; doc.delete())
            console.log('bookmark deleted')
          } catch (error) {
            console.error('error deleteing post.', error)
          }
  </code></pre>
  <p>Problem: I get the following console error when trying to delete the bookmark:</p>
  <pre><code>EditPost.vue?4144:107 error deleteing post. TypeError: doc.delete is not a function
      at eval (EditPost.vue?4144:95)
      at eval (prebuilt-28889c43-f8ea673b.js?f270:18228)
      at eval (prebuilt-28889c43-f8ea673b.js?f270:16453)
      at eval (prebuilt-28889c43-f8ea673b.js?f270:11563)
      at t.inorderTraversal (prebuilt-28889c43-f8ea673b.js?f270:2801)
      at t.inorderTraversal (prebuilt-28889c43-f8ea673b.js?f270:2719)
      at t.forEach (prebuilt-28889c43-f8ea673b.js?f270:11562)
      at t.forEach (prebuilt-28889c43-f8ea673b.js?f270:16452)
      at t.forEach (prebuilt-28889c43-f8ea673b.js?f270:18227)
      at _callee$ (EditPost.vue?4144:94)
  </code></pre>
  <p>How can I correctly write this delete logic? Thanks!</p>