[TEST] Remove out of date tests. (elastic/x-pack-elasticsearch#1078)

The path has changed so it’s no longer possible to distinguish between data feed and job tasks.
The preceding test get_datafeed provides ample coverage anyway.

Original commit: elastic/x-pack-elasticsearch@780b1beb6b

plugin/src/test/resources/rest-api-spec/test/ml/start_stop_datafeed.yaml

@@ -74,12 +74,6 @@ setup:
         datafeed_id: "datafeed-1"
   - match: { datafeeds.0.state: started }
 
-  - do:
-      cluster.state:
-        metric: [ metadata ]
-        filter_path: metadata.persistent_tasks.**.DatafeedState
-  - match: {metadata.persistent_tasks.tasks.0.status.DatafeedState.state: started}
-
   - do:
       xpack.ml.stop_datafeed:
         "datafeed_id": "datafeed-1"
@@ -89,12 +83,6 @@ setup:
         datafeed_id: "datafeed-1"
   - match: { datafeeds.0.state: stopped }
 
-  - do:
-      cluster.state:
-        metric: [ metadata ]
-        filter_path: metadata.persistent_tasks.**.DatafeedState
-  - is_false: metadata.persistent_tasks.tasks.0.status.DatafeedState
-
 ---
 "Test start datafeed given start is now":
   - do: