RFR: 8274069: Clean up g1ParScanThreadState a bit

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that does some minor refactoring for issues found in JDK-8271880/PR#5037:

- remove declaration without definition
- move inlined code into hpp file
- factor out some code

Testing: gha, local compilation, local gc/g1

Thanks,
  Thomas

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Commit messages:
 - Clean up

Changes: https://git.openjdk.java.net/jdk/pull/5607/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5607&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274069
  Stats: 67 lines in 3 files changed: 35 ins; 23 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5607.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5607/head:pull/5607

PR: https://git.openjdk.java.net/jdk/pull/5607