Remove redundant field SeekableStreamIndexTask.authorizerMapper

[kfaraz]

Description

#10308 had made a change to start assigning the SeekableStreamIndexTaskRunner.authorizerMapper
in the runTask method rather than the constructor, but that PR missed removing it from the constructor
and it has just been dangling since then.

Spotbugs seems to have finally found this issue.

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This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.