[Doc] Removed Mentions of the New Backend System

Signed-off-by: Andre Wanlin <awanlin@spotify.com>

docs/backend-system/architecture/01-index.md

@@ -2,12 +2,12 @@
 id: index
 title: Backend System Architecture
 sidebar_label: Overview
-description: The structure and architecture of the new Backend System and its component parts
+description: The structure and architecture of the Backend System and its component parts
 ---
 
 ## Building Blocks
 
-This section introduces the high-level building blocks upon which this new
+This section introduces the high-level building blocks upon which this
 system is built. These are all concepts that exist in our current system in one
 way or another, but they have all been lifted up to be first class concerns in
 the new system. Regardless of whether you are setting up your own backstage

docs/backend-system/building-backends/01-index.md

@@ -2,7 +2,7 @@
 id: index
 title: Building Backends
 sidebar_label: Overview
-description: Building backends using the new backend system
+description: Building backends using the backend system
 ---
 
 :::note Note

docs/backend-system/building-plugins-and-modules/01-index.md

@@ -2,16 +2,9 @@
 id: index
 title: Building Backend Plugins and Modules
 sidebar_label: Overview
-description: Building backend plugins and modules using the new backend system
+description: Building backend plugins and modules using the backend system
 ---
 
-:::note Note
-
-If you have an existing backend and/or backend plugins that are not yet
-using the new backend system, see [migrating](./08-migrating.md).
-
-:::
-
 This section covers how to build your own backend [plugins](../architecture/04-plugins.md) and
 [modules](../architecture/06-modules.md). They are sometimes collectively referred to as
 backend _features_, and are the building blocks that adopters add to their