docs/backend-system: remove old doc from sidebar

Co-authored-by: Fredrik Adelöw <freben@gmail.com>
Co-authored-by: Johan Haals <johan.haals@gmail.com>
Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

microsite/sidebars.json

@@ -238,8 +238,7 @@
           "plugins/proxying",
           "plugins/backend-plugin",
           "plugins/call-existing-api",
-          "plugins/url-reader",
-          "plugins/new-backend-system"
+          "plugins/url-reader"
         ]
       },
       {

mkdocs.yml

@@ -138,7 +138,6 @@ nav:
           - Backend plugin: 'plugins/backend-plugin.md'
           - Call existing API: 'plugins/call-existing-api.md'
           - URL Reader: 'plugins/url-reader.md'
-          - New Backend System: 'plugins/new-backend-system.md'
       - Testing:
           - Testing with Jest: 'plugins/testing.md'
       - Publishing: