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

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
 ```

appengine.go

@@ -1,25 +0,0 @@
-// Go MySQL Driver - A MySQL-Driver for Go's database/sql package
-//
-// Copyright 2013 The Go-MySQL-Driver Authors. All rights reserved.
-//
-// This Source Code Form is subject to the terms of the Mozilla Public
-// License, v. 2.0. If a copy of the MPL was not distributed with this file,
-// You can obtain one at http://mozilla.org/MPL/2.0/.
-
-// +build appengine
-
-package mysql
-
-import (
-	"context"
-	"net"
-
-	"google.golang.org/appengine/cloudsql"
-)
-
-func init() {
-	RegisterDialContext("cloudsql", func(_ context.Context, instance string) (net.Conn, error) {
-		// XXX: the cloudsql driver still does not export a Context-aware dialer.
-		return cloudsql.Dial(instance)
-	})
-}