Remove the task graph and the tool that produces it

Change-Id: I8fa4b1e1646f4072fafa4521648f0ec158e4cebc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/95561
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

Author: bwilkerson