Remove Cloud SQL dialer (#1007)

CloudSQL is only available up to Go 1.9 on Google AppEngine, which was phased out. Starting from 2019-10-01, no new apps can be deployed to GAE/Go 1.9 anymore. This dialer is thus obsolete.

Fixes #1002

Author: julienschmidt

README.md

@@ -396,12 +396,7 @@ TCP on a remote host, e.g. Amazon RDS:
 id:password@tcp(your-amazonaws-uri.com:3306)/dbname
 ```
 
-Google Cloud SQL on App Engine (First Generation MySQL Server):
-```
-user@cloudsql(project-id:instance-name)/dbname
-```
-
-Google Cloud SQL on App Engine (Second Generation MySQL Server):
+Google Cloud SQL on App Engine:
 ```
 user:password@unix(/cloudsql/project-id:region-name:instance-name)/dbname
 ```