[DOCS] Remove docs for the deprecated 'Java API'

[tlfeng]

Issue #, if available:
#142
Description of changes:

The PR requests merging the code into oss-docs branch.

• Remove the whole folder docs/java-api, which is used for the guide in this page: https://www.elastic.co/guide/en/elasticsearch/client/java-api/7.10/index.html, and it describes a deprecated Java client.

Note: the TransportClient that used by the "Java API" will be removed in Elasticsearch 8.0 (elastic/elasticsearch@b15d62c)

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[setiah]

Don't think we should remove these docs. The transport client is still present in 7.10 and docs are needed. Whether we remove it in future or not will be a decision on opensearch community.