Revert "improve fn & on undefined callback message"

This reverts commit f6f9f52.

The changes landed cause failures in CI (I should have rebased the
branch to make sure that the code still worked after the last few
glimmer-vm updates).

```
[0413/140947.176661:INFO:CONSOLE(52)] "    Failed assertion: Promise rejected during " model is tracked": args.positional.at(...).debug is not a function
TypeError: args.positional.at(...).debug is not a function
    at OnModifierState.updateFromArgs (http://localhost:13141/tests/ember.js:9767:68)
    at OnModifierManager.install (http://localhost:13141/tests/ember.js:10002:13)
    at TransactionImpl.commit (http://localhost:13141/tests/ember.js:48873:19)
    at EnvironmentImpl.commit (http://localhost:13141/tests/ember.js:49009:19)
    at inTransaction (http://localhost:13141/tests/ember.js:49167:13)
    at InteractiveRenderer._renderRoots (http://localhost:13141/tests/ember.js:11540:37)
    at InteractiveRenderer._renderRootsTransaction (http://localhost:13141/tests/ember.js:11601:14)
    at InteractiveRenderer._renderRoot (http://localhost:13141/tests/ember.js:11527:12)
    at InteractiveRenderer._appendDefinition (http://localhost:13141/tests/ember.js:11442:12)
    at InteractiveRenderer.appendOutletView (http://localhost:13141/tests/ember.js:11428:12)", source:  (52)
```

(cherry picked from commit 109566a)

packages/@ember/-internals/glimmer/lib/helpers/fn.ts

@@ -85,9 +85,8 @@ function fn({ positional }: CapturedArguments) {
   if (DEBUG && typeof callbackRef[INVOKE] !== 'function') {
     let callback = callbackRef.value();
 
-    const debug = (<any>callbackRef).debug && (<any>callbackRef).debug();
     assert(
-      `You must pass a function as the \`fn\` helpers first argument, you passed ${debug} to \`fn\` but it was ${callback}`,
+      `You must pass a function as the \`fn\` helpers first argument, you passed ${callback}`,
       typeof callback === 'function'
     );
   }

packages/@ember/-internals/glimmer/lib/modifiers/on.ts

@@ -100,15 +100,11 @@ export class OnModifierState {
       this.eventName = eventName;
       this.shouldUpdate = true;
     }
-    if (DEBUG) {
-      const debug = args.positional.at(1) && (<any>args.positional.at(1)).debug();
-      const value = args.positional.at(1) && args.positional.at(1).value();
-      assert(
-        `You must pass a function as the second argument to the \`on\` modifier, you passed ${debug} to \`on\` but it was ${value}`,
-        value !== undefined && typeof value === 'function'
-      );
-    }
 
+    assert(
+      'You must pass a function as the second argument to the `on` modifier',
+      args.positional.at(1) !== undefined && typeof args.positional.at(1).value() === 'function'
+    );
     let userProvidedCallback = args.positional.at(1).value() as EventListener;
     if (userProvidedCallback !== this.userProvidedCallback) {
       this.userProvidedCallback = userProvidedCallback;

packages/@ember/-internals/glimmer/tests/integration/helpers/fn-test.js

@@ -127,7 +127,7 @@ moduleFor(
           arg1: 'foo',
           arg2: 'bar',
         });
-      }, /You must pass a function as the `fn` helpers first argument, you passed this.myFunc to `fn` but it was null/);
+      }, /You must pass a function as the `fn` helpers first argument, you passed null/);
     }
 
     '@test asserts if the provided function accesses `this` without being bound prior to passing to fn'(