Avoid private cudf DeviceScalar in favor of using pylibcudf & pyarrow

[mroeschke]

Description

In cudf, I'm looking to ultimately remove the DeviceScalar implementation in favor of using pylibcudf.Scalar and pyarrow.

Before, it appears there was a cudf helper from getting an element from a string column's child (offset) column. There now exists pylibcudf APIs to do this and therefore can avoid having to directly access DeviceScalar

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• I am familiar with the Contributing Guidelines.
• When the PR is ready for review, new or existing tests cover these changes.
• When the PR is ready for review, the documentation is up to date with these changes.

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