Remove ILM's CleanupTargetIndexStep (elastic#129130)

This step was added to ILM when downsampling was not yet retryable.
Downsampling has been retryable for some time now, so there were no
usages for this step.
We still keep the step name (and use it for a no-op step) for BwC.

Author: nielsbauman

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ilm/CleanupTargetIndexStep.java

@@ -52,6 +52,7 @@ public class DownsampleAction implements LifecycleAction {
     public static final TimeValue DEFAULT_WAIT_TIMEOUT = new TimeValue(1, TimeUnit.DAYS);
     private static final ParseField FIXED_INTERVAL_FIELD = new ParseField(DownsampleConfig.FIXED_INTERVAL);
     private static final ParseField WAIT_TIMEOUT_FIELD = new ParseField("wait_timeout");
+    static final String BWC_CLEANUP_TARGET_INDEX_NAME = "cleanup-target-index";
 
     private static final ConstructingObjectParser<DownsampleAction, Void> PARSER = new ConstructingObjectParser<>(
         NAME,
@@ -141,7 +142,7 @@ public List<Step> toSteps(Client client, String phase, StepKey nextStepKey) {
         StepKey waitForNoFollowerStepKey = new StepKey(phase, NAME, WaitForNoFollowersStep.NAME);
         StepKey waitTimeSeriesEndTimePassesKey = new StepKey(phase, NAME, WaitUntilTimeSeriesEndTimePassesStep.NAME);
         StepKey readOnlyKey = new StepKey(phase, NAME, ReadOnlyStep.NAME);
-        StepKey cleanupDownsampleIndexKey = new StepKey(phase, NAME, CleanupTargetIndexStep.NAME);
+        StepKey cleanupDownsampleIndexKey = new StepKey(phase, NAME, BWC_CLEANUP_TARGET_INDEX_NAME);
         StepKey generateDownsampleIndexNameKey = new StepKey(phase, NAME, DownsamplePrepareLifeCycleStateStep.NAME);
         StepKey downsampleKey = new StepKey(phase, NAME, DownsampleStep.NAME);
         StepKey waitForDownsampleIndexKey = new StepKey(phase, NAME, WaitForIndexColorStep.NAME);

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ilm/DownsampleAction.java

@@ -99,7 +99,7 @@ public void testToSteps() {
         assertThat(steps.get(4).getNextStepKey().name(), equalTo(DownsamplePrepareLifeCycleStateStep.NAME));
 
         assertTrue(steps.get(5) instanceof NoopStep);
-        assertThat(steps.get(5).getKey().name(), equalTo(CleanupTargetIndexStep.NAME));
+        assertThat(steps.get(5).getKey().name(), equalTo(DownsampleAction.BWC_CLEANUP_TARGET_INDEX_NAME));
         assertThat(steps.get(5).getNextStepKey().name(), equalTo(DownsampleStep.NAME));
 
         assertTrue(steps.get(6) instanceof DownsamplePrepareLifeCycleStateStep);