[Observability] Remove search attributes is missing in Java

[rachfop]

Remove search attributes is missing is Java

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You can provide a code snippet in the comments or a link to the line numbers of the samples-repo. Once added, I'll update the app-dev page with the new code snippet.

[Spikhalskiy]

To remove a SA in Java, io.temporal.common.SearchAttribute#UNSET_VALUE value can be used in the upsert map (see https://github.com/temporalio/documentation/issues/1477) for the SA name.