RFR: 8280079: Serial: Remove empty Generation::prepare_for_verify

[Albert Mingkun Yang]

Simple change of removing some empty methods.

Test: hotspot_gc

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Commit messages:
 - prepare_for_verify

Changes: https://git.openjdk.java.net/jdk/pull/7112/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7112&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280079
  Stats: 22 lines in 6 files changed: 0 ins; 22 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7112.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7112/head:pull/7112

PR: https://git.openjdk.java.net/jdk/pull/7112