Remove Goma support from building Dart for testing on the bots.

This change is safe to land once RBE is supported on beta & stable:

https://dart-review.googlesource.com/c/sdk/+/357342
https://dart-review.googlesource.com/c/sdk/+/357324

Bug: b/296994239
Change-Id: I402772476f2a82310ab1b0eb0f458a76cfd6ed9f
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/357680
Reviewed-by: William Hesse <whesse@google.com>
Commit-Queue: Jonas Termansen <sortie@google.com>
21 files changed