Remove redundant DCO workflow (#8188)
We added [our own DCO workflow](#8086) was added because the official
DCO App did not support GitHub merge queues. The official App has now
shipped `merge_group` support in
[dcoapp/app#292](https://github.com/dcoapp/app/pull/292). So we don't
need our own workflow anymore.
This PR removes the merge queue DCO workflow.
Signed-off-by: Masahiro Tanaka <mtanaka@anyscale.com>
