refactor(controller): stop auto-setting RUNNER_FEATURE_FLAG_EPHEMERAL (#1385)

This feature flag was provided from ARC to runner container automatically to let it use `--ephemeral` instead of `--once` by default. As the support for `--once` is being dropped from the runner image via #1384, we no longer need that.

Ref #1196

acceptance/testdata/runnerdeploy.envsubst.yaml

@@ -19,11 +19,6 @@ spec:
 
       ephemeral: ${TEST_EPHEMERAL}
 
-      # Whether to pass --ephemeral (true) or --once (false, deprecated)
-      env:
-      - name: RUNNER_FEATURE_FLAG_EPHEMERAL
-        value: "${RUNNER_FEATURE_FLAG_EPHEMERAL}"
-
       #
       # dockerd within runner container
       #

acceptance/testdata/runnerset.envsubst.yaml

@@ -59,9 +59,6 @@ spec:
       containers:
       - name: runner
         imagePullPolicy: IfNotPresent
-        env:
-        - name: RUNNER_FEATURE_FLAG_EPHEMERAL
-          value: "${RUNNER_FEATURE_FLAG_EPHEMERAL}"
       #- name: docker
       #  #image: mumoshu/actions-runner-dind:dev
 ---