Removed duplicate MONGODB_URI check in example (#42518)

This PR removes the duplicate check if the MONGODB_URI environment variable is set.

examples/with-mongodb/lib/mongodb.ts

@@ -1,7 +1,7 @@
 import { MongoClient } from 'mongodb'
 
 if (!process.env.MONGODB_URI) {
-  throw new Error('Invalid environment variable: "MONGODB_URI"')
+  throw new Error('Invalid/Missing environment variable: "MONGODB_URI"')
 }
 
 const uri = process.env.MONGODB_URI
@@ -10,10 +10,6 @@ const options = {}
 let client
 let clientPromise: Promise<MongoClient>
 
-if (!process.env.MONGODB_URI) {
-  throw new Error('Please add your Mongo URI to .env.local')
-}
-
 if (process.env.NODE_ENV === 'development') {
   // In development mode, use a global variable so that the value
   // is preserved across module reloads caused by HMR (Hot Module Replacement).