RFR: 8348898: Remove unused OctalDigits to clean up code

[Shaojin Wen]

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

-------------

Commit messages:
 - remove OctalDigits

Changes: https://git.openjdk.org/jdk/pull/23338/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23338&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348898
  Stats: 130 lines in 1 file changed: 0 ins; 130 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/23338.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23338/head:pull/23338

PR: https://git.openjdk.org/jdk/pull/23338