Remove GCE's 'jessie-backports.yml'

This file was duplicated, as there was never a GCE Jessie image with
backports.

This closes #310.

manifests/official/gce/jessie-backports.yml

@@ -1,31 +0,0 @@
----
-name: disk
-provider:
-  name: gce
-  description: Debian {system.release} {system.architecture}
-bootstrapper:
-  workspace: /target
-system:
-  release: jessie
-  architecture: amd64
-  bootloader: grub
-  charmap: UTF-8
-  locale: en_US
-  timezone: UTC
-volume:
-  backing: raw
-  partitions:
-    type: msdos
-    root:
-      filesystem: ext4
-      size: 10GiB
-packages:
-  install:
-    - rsync
-    - screen
-    - vim
-plugins:
-  google_cloud_sdk: {}
-  ntp:
-    servers:
-    - metadata.google.internal