Cleanup old releases

Cleanup docs for Prometheus 2.0 -> 2.2.
Signed-off-by: Ben Kochie <superq@gmail.com>

nanoc.yaml

@@ -110,36 +110,6 @@ data_sources:
       refspec: release-2.3
       canonical: /docs/prometheus/latest/
       outdated: /docs/prometheus/latest/
-  -
-    type: repo_docs
-    identifier_type: legacy
-    items_root: /docs/prometheus/2.2/
-    config:
-      name: '2.2'
-      repository: https://github.com/prometheus/prometheus.git
-      refspec: release-2.2
-      canonical: /docs/prometheus/latest/
-      outdated: /docs/prometheus/latest/
-  -
-    type: repo_docs
-    identifier_type: legacy
-    items_root: /docs/prometheus/2.1/
-    config:
-      name: '2.1'
-      repository: https://github.com/prometheus/prometheus.git
-      refspec: release-2.1
-      canonical: /docs/prometheus/latest/
-      outdated: /docs/prometheus/latest/
-  -
-    type: repo_docs
-    identifier_type: legacy
-    items_root: /docs/prometheus/2.0/
-    config:
-      name: '2.0'
-      repository: https://github.com/prometheus/prometheus.git
-      refspec: release-2.0
-      canonical: /docs/prometheus/latest/
-      outdated: /docs/prometheus/latest/
   -
     type: repo_docs
     identifier_type: legacy