[ILM] change remove-policy-from-index http method from DELETE to POST (#35268)

The remove-ilm-from-index API was using the DELETE http method
to signify that something is being removed. Although, metadata
about ILM for the index is being deleted, no entity/resource
is being deleted during this operation. POST is more in line with
what this API is actually doing, it is modifying the metadata for
an index. As part of this change, `remove` is also appended to the path 
to be more explicit about its actions.

client/rest-high-level/src/main/java/org/elasticsearch/client/IndexLifecycleRequestConverters.java

@@ -78,10 +78,10 @@ final class IndexLifecycleRequestConverters {
     static Request removeIndexLifecyclePolicy(RemoveIndexLifecyclePolicyRequest removePolicyRequest) {
         String[] indices = removePolicyRequest.indices() == null ?
                 Strings.EMPTY_ARRAY : removePolicyRequest.indices().toArray(new String[] {});
-        Request request = new Request(HttpDelete.METHOD_NAME,
+        Request request = new Request(HttpPost.METHOD_NAME,
                 new RequestConverters.EndpointBuilder()
                         .addCommaSeparatedPathParts(indices)
-                        .addPathPartAsIs("_ilm")
+                        .addPathPartAsIs("_ilm", "remove")
                         .build());
         RequestConverters.Params params = new RequestConverters.Params(request);
         params.withIndicesOptions(removePolicyRequest.indicesOptions());

client/rest-high-level/src/test/java/org/elasticsearch/client/IndexLifecycleRequestConvertersTests.java

@@ -99,9 +99,9 @@ public class IndexLifecycleRequestConvertersTests extends ESTestCase {
         setRandomMasterTimeout(req::setMasterTimeout, TimedRequest.DEFAULT_MASTER_NODE_TIMEOUT, expectedParams);
 
         Request request = IndexLifecycleRequestConverters.removeIndexLifecyclePolicy(req);
-        assertThat(request.getMethod(), equalTo(HttpDelete.METHOD_NAME));
+        assertThat(request.getMethod(), equalTo(HttpPost.METHOD_NAME));
         String idxString = Strings.arrayToCommaDelimitedString(indices);
-        assertThat(request.getEndpoint(), equalTo("/" + (idxString.isEmpty() ? "" : (idxString + "/")) + "_ilm"));
+        assertThat(request.getEndpoint(), equalTo("/" + (idxString.isEmpty() ? "" : (idxString + "/")) + "_ilm/remove"));
         assertThat(request.getParameters(), equalTo(expectedParams));
     }
 

docs/reference/ilm/apis/remove-policy.asciidoc

@@ -10,7 +10,7 @@ Unassigns a policy from a specified index pattern
 
 ==== Request
 
-`DELETE <index>/_ilm`
+`POST <index>/_ilm/remove`
 
 ==== Description
 
@@ -80,7 +80,7 @@ PUT my_index
 
 [source,js]
 --------------------------------------------------
-DELETE my_index/_ilm
+POST my_index/_ilm/remove
 --------------------------------------------------
 // CONSOLE
 // TEST[continued]

x-pack/plugin/ilm/src/main/java/org/elasticsearch/xpack/indexlifecycle/action/RestRemoveIndexLifecyclePolicyAction.java

@@ -22,7 +22,7 @@ public class RestRemoveIndexLifecyclePolicyAction extends BaseRestHandler {
 
     public RestRemoveIndexLifecyclePolicyAction(Settings settings, RestController controller) {
         super(settings);
-        controller.registerHandler(RestRequest.Method.DELETE, "/{index}/_ilm", this);
+        controller.registerHandler(RestRequest.Method.POST, "/{index}/_ilm/remove", this);
     }
 
     @Override

x-pack/plugin/src/test/resources/rest-api-spec/api/ilm.remove_policy.json

@@ -1,10 +1,10 @@
 {
   "ilm.remove_policy": {
     "documentation": "http://www.elastic.co/guide/en/index_lifecycle/current/index_lifecycle.html",
-    "methods": [ "DELETE" ],
+    "methods": [ "POST" ],
     "url": {
-      "path": "/{index}/_ilm",
-      "paths": ["/{index}/_ilm", "/_ilm"],
+      "path": "/{index}/_ilm/remove",
+      "paths": ["/{index}/_ilm/remove", "/_ilm/remove"],
       "parts": {
         "index": {
           "type" : "string",