Remove unused DEFAULT_HELPER_MANAGER feature flag

This flag was always set to `true` and never actually read anywhere —
the FEATURES property was defined on the GlobalContext interface and
set during initialization, but no code ever consumed it. Remove the
flag, the FEATURES property from the interface, and the initialization.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: NullVoxPopuli

packages/@ember/-internals/glimmer/lib/environment.ts

@@ -19,10 +19,6 @@ import toBool from './utils/to-bool';
 // Setup global context
 
 setGlobalContext({
-  FEATURES: {
-    DEFAULT_HELPER_MANAGER: true,
-  },
-
   scheduleRevalidate() {
     _backburner.ensureInstance();
   },

packages/@glimmer/global-context/index.ts

@@ -165,9 +165,6 @@ export interface GlobalContext {
       id: string;
     }
   ) => void;
-  FEATURES?: {
-    DEFAULT_HELPER_MANAGER?: boolean;
-  };
 }
 
 let globalContextWasSet = false;