RFR: 8315933: Serial: Remove empty Generation::ensure_parsability

[Albert Mingkun Yang]

Simple removing redundant code.

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Commit messages:
 - s1-parse

Changes: https://git.openjdk.org/jdk/pull/15636/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15636&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315933
  Stats: 20 lines in 3 files changed: 0 ins; 20 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15636.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15636/head:pull/15636

PR: https://git.openjdk.org/jdk/pull/15636