[PURIFY] Remove DataStreams

[tlfeng]

Remove "Data Steams" codes, according to the discussion in PR #36

Basically revert the PRs: #28 #37

[tlfeng]

A sub issue, should be resolved after cleanup for DataStreams:

Remove codes related to "Index Lifecycle Management" altogether from "DataStreams" classes.

ILM codes were removed in the PR #20, but some leftover were introduced by bring back DataStream in th PR #28

As far as I can see, there are a bunch of codes about ilmPolicy in DataStream and GetDataStreamAction classes.

The test failure should be disappear after the issue is resolved.

REPRODUCE WITH: ./gradlew ':client:rest-high-level:test' --tests "org.elasticsearch.client.indices.GetDataStreamResponseTests.testFromXContent" -Dtests.seed=5E1553B2DED480C5 -Dtests.security.manager=true -Dtests.locale=es-AR -Dtests.timezone=America/Punta_Arenas -Druntime.java=15

org.elasticsearch.client.indices.GetDataStreamResponseTests > testFromXContent FAILED
    org.elasticsearch.common.xcontent.XContentParseException: [1:8288] [data_stream] unknown field [ilm_policy]
        at __randomizedtesting.SeedInfo.seed([5E1553B2DED480C5:7F7D66AA5773601F]:0)
        at org.elasticsearch.common.xcontent.ObjectParser.lambda$errorOnUnknown$2(ObjectParser.java:102)
        at org.elasticsearch.common.xcontent.ObjectParser.parse(ObjectParser.java:297)
        at org.elasticsearch.common.xcontent.ConstructingObjectParser.parse(ConstructingObjectParser.java:171)
        at org.elasticsearch.client.indices.DataStream.fromXContent(DataStream.java:116)
        at org.elasticsearch.client.indices.GetDataStreamResponse.fromXContent(GetDataStreamResponse.java:48)
        at org.elasticsearch.client.indices.GetDataStreamResponseTests.doParseToClientInstance(GetDataStreamResponseTests.java:75)
        at org.elasticsearch.client.indices.GetDataStreamResponseTests.doParseToClientInstance(GetDataStreamResponseTests.java:42)

[adnapibar]

The test failure is fixed by #63

[adnapibar]

Closing this issue as the tests were fixed by removing the ILM policy field #63 .