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'

.crow/build-all-versions.yaml

@@ -1,7 +1,7 @@
 ### Manual full rebuild of all CRAN binaries for one OS/arch.
 # Set the variables below in the manual-run form (or via --var on the CLI).
-# `target_arch` routes the matrix; OS/OS_VERSION/R_VERSION/K8S_INSTANCE_TYPE
+# `target_arch` routes the matrix; OS/OS_VERSION/R_VERSION select the platform
-# select the platform image, cache volume and node.
+# image and cache volume. Placement is via the agent label (artemis/gaia).
 # Skip list lives in local/excluded-packages.json (read by local/build-all.R).
 variables:
   target_arch:
@@ -22,9 +22,6 @@ 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
@@ -40,49 +37,41 @@ matrix:
   include:
     - ARCH: amd64
       AGENT: artemis
-      BACKEND: kubernetes
       R_PKG_CACHE_DIR: ''
       SPLIT_INTO: 4
       SPLIT_INDEX: 1
     - ARCH: amd64
       AGENT: artemis
-      BACKEND: kubernetes
       R_PKG_CACHE_DIR: ''
       SPLIT_INTO: 4
       SPLIT_INDEX: 2
     - ARCH: amd64
       AGENT: artemis
-      BACKEND: kubernetes
       R_PKG_CACHE_DIR: ''
       SPLIT_INTO: 4
       SPLIT_INDEX: 3
     - ARCH: amd64
       AGENT: artemis
-      BACKEND: kubernetes
       R_PKG_CACHE_DIR: ''
       SPLIT_INTO: 4
       SPLIT_INDEX: 4
     - ARCH: arm64
       AGENT: gaia
-      BACKEND: docker
       R_PKG_CACHE_DIR: /mnt/cache/pkgcache
       SPLIT_INTO: 4
       SPLIT_INDEX: 1
     - ARCH: arm64
       AGENT: gaia
-      BACKEND: docker
       R_PKG_CACHE_DIR: /mnt/cache/pkgcache
       SPLIT_INTO: 4
       SPLIT_INDEX: 2
     - ARCH: arm64
       AGENT: gaia
-      BACKEND: docker
       R_PKG_CACHE_DIR: /mnt/cache/pkgcache
       SPLIT_INTO: 4
       SPLIT_INDEX: 3
     - ARCH: arm64
       AGENT: gaia
-      BACKEND: docker
       R_PKG_CACHE_DIR: /mnt/cache/pkgcache
       SPLIT_INTO: 4
       SPLIT_INDEX: 4
@@ -144,22 +133,6 @@ steps:
           limits:
             memory: 20Gi
             cpu: 2000m
-      kubernetes:
-        resources:
-          requests:
-            memory: 5Gi
-            cpu: 1000m
-          limits:
-            memory: 20Gi
-            cpu: 2000m
-        nodeSelector:
-          kubernetes.io/arch: ${ARCH}
-          node.kubernetes.io/instance-type: ${K8S_INSTANCE_TYPE}
-        tolerations:
-          - key: 'CI'
-            operator: 'Equal'
-            value: 'true'
-            effect: 'NoSchedule'
   - name: 'Upload package indexes'
     image: 'reg.devxy.io/rpkgs/build-env-${OS}:${OS_VERSION}'
     pull: true
@@ -187,20 +160,4 @@ steps:
             cpu: 1000m
           limits:
             memory: 20Gi
-            cpu: 2000m
+            cpu: 2000m
-      kubernetes:
-        resources:
-          requests:
-            memory: 5Gi
-            cpu: 1000m
-          limits:
-            memory: 20Gi
-            cpu: 2000m
-        nodeSelector:
-          kubernetes.io/arch: ${ARCH}
-          node.kubernetes.io/instance-type: ${K8S_INSTANCE_TYPE}
-        tolerations:
-          - key: 'CI'
-            operator: 'Equal'
-            value: 'true'
-            effect: 'NoSchedule'