Remove Stein CI jobs

Stein release of Octavia has reached end-of-life (EOL) support [1].
The stable/stein Octavia branch no longer accepts changes.
We can now remove the Stein CI jobs.

[1] https://review.opendev.org/c/openstack/releases/+/772847

Change-Id: I14ab0f8b159fdd0c7e81b94297a57f30260579fa
2 files changed