kubernetes-csi: avoid overriding Kubernetes version for single-Kubernetes repos

The original intention was to let repos which only need to be tested
against a single Kubernetes release choose that release
themselves. The comment for `single_kubernetes_repos` still describes
that behavior.

kubernetes#13434 changed that so
that 1.15 is used for all of these repos because of some issues with
external-snapshotter.

external-snapshotter now has 1.15 in its own prow.sh, so we don't need
the override anymore in the test job and can revert to the original
behavior.

Author: pohly

config/jobs/kubernetes-csi/external-resizer/external-resizer-config.yaml

@@ -25,8 +25,6 @@ presubmits:
         args:
         - ./.prow.sh
         env:
-        - name: CSI_PROW_KUBERNETES_VERSION
-          value: "1.15.0"
         - name: CSI_PROW_TESTS
           value: "sanity serial parallel"
         # docker-in-docker needs privileged mode
@@ -58,8 +56,6 @@ presubmits:
         args:
         - ./.prow.sh
         env:
-        - name: CSI_PROW_KUBERNETES_VERSION
-          value: "1.15.0"
         - name: CSI_PROW_TESTS
           value: "unit"
         # docker-in-docker needs privileged mode
@@ -91,8 +87,6 @@ presubmits:
         args:
         - ./.prow.sh
         env:
-        - name: CSI_PROW_KUBERNETES_VERSION
-          value: "1.15.0"
         - name: CSI_PROW_TESTS
           value: "serial-alpha parallel-alpha"
         # docker-in-docker needs privileged mode

config/jobs/kubernetes-csi/external-snapshotter/external-snapshotter-config.yaml

@@ -25,8 +25,6 @@ presubmits:
         args:
         - ./.prow.sh
         env:
-        - name: CSI_PROW_KUBERNETES_VERSION
-          value: "1.15.0"
         - name: CSI_PROW_TESTS
           value: "sanity serial parallel"
         # docker-in-docker needs privileged mode
@@ -58,8 +56,6 @@ presubmits:
         args:
         - ./.prow.sh
         env:
-        - name: CSI_PROW_KUBERNETES_VERSION
-          value: "1.15.0"
         - name: CSI_PROW_TESTS
           value: "unit"
         # docker-in-docker needs privileged mode
@@ -91,8 +87,6 @@ presubmits:
         args:
         - ./.prow.sh
         env:
-        - name: CSI_PROW_KUBERNETES_VERSION
-          value: "1.15.0"
         - name: CSI_PROW_TESTS
           value: "serial-alpha parallel-alpha"
         # docker-in-docker needs privileged mode

config/jobs/kubernetes-csi/gen-jobs.sh

@@ -332,8 +332,6 @@ EOF
         args:
         - ./.prow.sh
         env:
-        - name: CSI_PROW_KUBERNETES_VERSION
-          value: "${latest_stable_k8s_version}"
         - name: CSI_PROW_TESTS
           value: "$(expand_tests "$tests")"
         # docker-in-docker needs privileged mode

config/jobs/kubernetes-csi/livenessprobe/livenessprobe-config.yaml

@@ -25,8 +25,6 @@ presubmits:
         args:
         - ./.prow.sh
         env:
-        - name: CSI_PROW_KUBERNETES_VERSION
-          value: "1.15.0"
         - name: CSI_PROW_TESTS
           value: "sanity serial parallel"
         # docker-in-docker needs privileged mode
@@ -58,8 +56,6 @@ presubmits:
         args:
         - ./.prow.sh
         env:
-        - name: CSI_PROW_KUBERNETES_VERSION
-          value: "1.15.0"
         - name: CSI_PROW_TESTS
           value: "unit"
         # docker-in-docker needs privileged mode
@@ -91,8 +87,6 @@ presubmits:
         args:
         - ./.prow.sh
         env:
-        - name: CSI_PROW_KUBERNETES_VERSION
-          value: "1.15.0"
         - name: CSI_PROW_TESTS
           value: "serial-alpha parallel-alpha"
         # docker-in-docker needs privileged mode