RFR: 8280458: G1: Remove G1BlockOffsetTablePart::_next_offset_threshold

Fri Jan 21 15:00:16 UTC 2022

This PR consists of two commits:

1. the real change of removing the field and updating surrounding code.
2. refactoring only (rename, comment updates, method inline, etc).

PS: `oop.inline.hpp` is needed for `klass_or_null`; it worked before by accident. Changes to `G1FullGCCompactionPoint` is kept to minimal to avoid conflicts with #7106.

Test: tier1-6

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Commit messages:
 - rename
 - remove-bot-threshold

Changes: https://git.openjdk.java.net/jdk/pull/7177/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7177&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280458
  Stats: 145 lines in 10 files changed: 14 ins; 95 del; 36 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7177.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7177/head:pull/7177

PR: https://git.openjdk.java.net/jdk/pull/7177