Revert "improve fn & on undefined callback message"

[rwjblue]

This reverts the changes from #18353.

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)