RFR: 8323738: Serial: Remove unreachable methods in Generation

[Albert Mingkun Yang]

Trivial removing effectively dead code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/17429/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17429&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323738
  Stats: 22 lines in 1 file changed: 0 ins; 22 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17429.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17429/head:pull/17429

PR: https://git.openjdk.org/jdk/pull/17429