[dart2js] Remove dart2js unit tests from the NNBD builder
[dart2js] Remove dart2js unit tests from the NNBD builder

These tests were added to test the forked NNBD SDK but the SDK has long
been unforked.

Bug: b/200928253
Change-Id: Idb0891c2141a09320bf7779c260c79ec868f0ad0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/214402
Auto-Submit: Alexander Thomas <29642742b6693024c89de8232f2e2542cf7eedf7@google.com>
Commit-Queue: Sigmund Cherem <beaea8fd7d2a56555046ba4f362c8a93e3938576@google.com>
Reviewed-by: Sigmund Cherem <beaea8fd7d2a56555046ba4f362c8a93e3938576@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk