title: How to delete a field in Firestore document with flutter
body: <p>I'm making a Flutter application.</p>
  
  <p>But, I cannot delete a field in the Firestore document.
  In another language I know to use <code>FieldValue.delete()</code> to delete a file in Firestorm document.</p>
  
  <p>In Dart, How do I delete?</p>