[Autoroller] Delete the Dart->Flutter/Engine roller
[Autoroller] Delete the Dart->Flutter/Engine roller

Was never used.

Bug: skia:
Change-Id: I346238997bd2e3914dd6cfaa69e4e00fdeff2152
Reviewed-on: https://skia-review.googlesource.com/c/173201
Reviewed-by: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>
Commit-Queue: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot