RFR: 8348898: Remove unused OctalDigits to clean up code

[Chen Liang]

On Tue, 28 Jan 2025 17:17:07 GMT, Shaojin Wen <swen at openjdk.org> wrote:

> This PR removes OctalDigits as it is no longer used after the removal of StringTemplate.

Removal of burdens is always good.

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Marked as reviewed by liach (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/23338#pullrequestreview-2579540810