Remove legacy support

[om26er]

We don't need the legacy support script anymore. Android Studio 4.0 enables old Android versions to use CompleteableFuture, so we can now support them without any scripts/hackery. Without any maintenance effort.

See https://developer.android.com/studio/releases#4-0-0-desugar

[om26er]

Sorry that was a false alarm. While Android Studio 4.0 did enable more Java8 APIs in Android, some are still missing for us to complete the transition. Some APIs for java.util.concurrent are supported but not all (yet). We'll revisit this once that's the case. Upstream issue https://issuetracker.google.com/issues/153794291