Remove prepare done job for DAG

What does this MR do?

Remove prepare done job for DAG

Why was this MR needed?

The prepare done job was added so that we can have jobs start as soon as the pipeline is created, since there wasn't a point to wait for the previous stage to finish. With needs we can specify needs: [] which does the same thing it will start the job imideally and doesn't wait for the previous stage as shown in https://docs.gitlab.com/ee/ci/yaml/#needs.

With this change we also solve a problem for docs/ only merge requests becuase prepare done wasn't running on those, which check feature flags (which runs on docs/ branches) required.

What's the best way to test this MR?

New pipeline just started, all jobs start running:

New DAG:

What are the relevant issue numbers?

Closes #26593 (closed)