[FEATURE EMBER_MODULE_UNIFICATION] Disable.

It is extremely unlikely that the implementation of module unification
that exists behind this feature flag will ever land, and due to the way
we test optional features the presence of this feature flag makes it
impossible to test other new features _without_ also testing module
unification (meaning the features can't be tested independently).

This disables the module unification feature completely such that it is
not possible to enable at runtime.

packages/@ember/canary-features/index.ts

@@ -15,7 +15,7 @@ import { assign } from '@ember/polyfills';
 export const DEFAULT_FEATURES = {
   EMBER_LIBRARIES_ISREGISTERED: null,
   EMBER_IMPROVED_INSTRUMENTATION: null,
-  EMBER_MODULE_UNIFICATION: null,
+  EMBER_MODULE_UNIFICATION: false,
   EMBER_METAL_TRACKED_PROPERTIES: null,
   EMBER_GLIMMER_FORWARD_MODIFIERS_WITH_SPLATTRIBUTES: true,
   EMBER_GLIMMER_ANGLE_BRACKET_BUILT_INS: true,