refactor(ci): drop K8S_INSTANCE_TYPE and kubernetes backend from build-all

Placement is handled by the agent label (artemis/gaia) and only the docker
backend is used, so the kubernetes backend_options blocks, the arch/instance
nodeSelector, and the K8S_INSTANCE_TYPE variable are dead. Remove them and the
now-redundant BACKEND matrix column.

.crow/build-all-versions-install-deps.yaml

@@ -21,9 +21,6 @@ variables:
       - 4.5.3
       - 4.4.3
     default: 4.5.3
-  K8S_INSTANCE_TYPE:
-    description: "k8s node instance-type (amd64/kubernetes only; ignored for arm64/docker)."
-    default: AX42
 
 when:
   - event: manual
@@ -76,16 +73,3 @@ steps:
           requests:
             memory: 5Gi
             cpu: 1000m
-      kubernetes:
-        resources:
-          requests:
-            memory: 5Gi
-            cpu: 1000m
-        nodeSelector:
-          kubernetes.io/arch: ${ARCH}
-          node.kubernetes.io/instance-type: ${K8S_INSTANCE_TYPE}
-        tolerations:
-          - key: 'CI'
-            operator: 'Equal'
-            value: 'true'
-            effect: 'NoSchedule'