Drop the manual Dart event delivery check

bindings/dart/nativeapi/test/ held only event_delivery_runtime.dart, a script
the macOS-only tools/tests/dart_event_delivery_macos.py drove against a C++
fixture, never a *_test.dart. Its presence made melos run `flutter test` in
nativeapi, which fails on a test directory without test files, so the Dart CI
test jobs failed on every platform since 1630105. Remove the script, the
harness and the fixture.

Author: lijy91