cmd/pushback: update deployment instructions

aedeploy is now bundled into "gcloud app".
Move it into the README now that it's a one liner.

Change-Id: I9bb0d16e94a062c561922c72cd02b0ee03f2c0c5
Reviewed-on: https://go-review.googlesource.com/76730
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cmd/pushback/README

@@ -1,6 +1,8 @@
-This service runs as a Google Managed VM.
+This service runs on Google App Engine flexible
 
-Deploy with ./deploy.bash.
+Deploy with:
+
+    gcloud app deploy --project symbolic-datum-552 --no-promote app.yaml
 
 Then use the Cloud Console to set the new version as the default,
 and delete the old instance (lest it keep running):

cmd/pushback/deploy.bash

@@ -1,4 +0,0 @@
-#!/bin/bash
-
-go get google.golang.org/appengine/cmd/aedeploy
-aedeploy gcloud --project symbolic-datum-552 app deploy --no-promote app.yaml