docs: remove premium notice (#47)

Author: jeanbza

stackdriver.go

@@ -38,7 +38,6 @@
 //   1. Create a Cloud project: https://support.google.com/cloud/answer/6251787?hl=en
 //   2. Enable billing: https://support.google.com/cloud/answer/6288653#new-billing
 //   3. Enable the Stackdriver Monitoring API: https://console.cloud.google.com/apis/dashboard
-//   4. Make sure you have a Premium Stackdriver account: https://cloud.google.com/monitoring/accounts/tiers
 //
 // These steps enable the API but don't require that your app is hosted on Google Cloud Platform.
 //