chore(datastore): delete invalid region tag 'ndb_flask' (#13202)

Author: eapl-gemugami

datastore/cloud-ndb/flask_app.py

@@ -13,7 +13,6 @@
 # limitations under the License.
 
 # [START datastore_ndb_flask]
-# [START ndb_flask]
 from flask import Flask
 
 from google.cloud import ndb
@@ -42,5 +41,4 @@ class Book(ndb.Model):
 def list_books():
     books = Book.query()
     return str([book.to_dict() for book in books])
-# [END ndb_flask]
 # [END datastore_ndb_flask]