google: remove legacy App Engine context stuff

Fixes golang/oauth2#276

Change-Id: I3f2bed7201f2e6bb58913f3fae2e64287b9a66d5
Reviewed-on: https://go-review.googlesource.com/c/161959
Reviewed-by: Ross Light <light@google.com>
Reviewed-by: Jean de Klerk <deklerk@google.com>
Reviewed-by: Chris Broadfoot <cbro@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

google/example_test.go

@@ -14,8 +14,6 @@ import (
 	"golang.org/x/oauth2"
 	"golang.org/x/oauth2/google"
 	"golang.org/x/oauth2/jwt"
-	"google.golang.org/appengine"
-	"google.golang.org/appengine/urlfetch"
 )
 
 func ExampleDefaultClient() {
@@ -122,20 +120,6 @@ func Example_serviceAccount() {
 	client.Get("...")
 }
 
-func ExampleAppEngineTokenSource() {
-	var req *http.Request // from the ServeHTTP handler
-	ctx := appengine.NewContext(req)
-	client := &http.Client{
-		Transport: &oauth2.Transport{
-			Source: google.AppEngineTokenSource(ctx, "https://www.googleapis.com/auth/bigquery"),
-			Base: &urlfetch.Transport{
-				Context: ctx,
-			},
-		},
-	}
-	client.Get("...")
-}
-
 func ExampleComputeTokenSource() {
 	client := &http.Client{
 		Transport: &oauth2.Transport{