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This reverts commit b567cf95.

Reason for revert: breaking official.continuous builders
BUG=751954

Original change's description:
> Add a "buildbucket" subcommand to mb.
> 
> This subcommand prints the trybot configuration in the
> buildbucket.config format expected by gerrit. This should hopefully
> make it a little easier to maintain the buildbucket.config file.
> 
> Bug: 
> Change-Id: I7d956efb6c08128ae0ceaa6b68bcc9001296549c
> Reviewed-on: https://chromium-review.googlesource.com/568666
> Reviewed-by: Dirk Pranke <dpranke@chromium.org>
> Commit-Queue: Peter Collingbourne <pcc@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#491647}

TBR=agable@chromium.org,dpranke@chromium.org,pcc@chromium.org

Change-Id: I2cc5f086b9787723337795c3c2323c3bafdc89f3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/599393Reviewed-by: Michael Moss <mmoss@chromium.org>
Commit-Queue: Michael Moss <mmoss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491656}