Remove openstacksdk from siblings job

Icc9adcbfae7a37c335ce4586741cb27d6e0f5c66 pins the openstacksdk
requirement to <0.99, but this fails when we try to use the zuul
checkout of master openstacksdk in the siblings job.

For now, we just need to drop it from the siblings job.

Change-Id: I6e755ef7d2fc95f7475315ed1be1c5d13e9d0f0b

.zuul.yaml

@@ -114,7 +114,6 @@
     required-projects:
       - zuul/nodepool
       - openstack/diskimage-builder
-      - openstack/openstacksdk
     requires:
       - python-builder-3.9-bullseye-container-image
       - python-base-3.9-bullseye-container-image
@@ -127,15 +126,12 @@
           target: nodepool-launcher
           tags:
             - siblings
-          siblings:
-            - opendev.org/openstack/openstacksdk
         - context: .
           repository: zuul/nodepool-builder
           target: nodepool-builder
           tags:
             - siblings
           siblings:
-            - opendev.org/openstack/openstacksdk
             - opendev.org/openstack/diskimage-builder
 
 - job: