[Intl] Removed non-working $fallback argument from ArrayAccessibleResourceBundle

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

The code in question didn't actually work. This was extracted from #9206.