RFR: 8074101: Add verification that all tasks are actually claimed during roots processing

[Albert Mingkun Yang]

The first commit removes some obsolete enum items, while the second commit adds the verification logic. Commit 2 introduces some "empty" task claims for the verification logic, explicitly marked in the comments.

Test: hotspot_gc

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Commit messages:
 - add claim verification
 - remove unused enum items

Changes: https://git.openjdk.java.net/jdk/pull/2046/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2046&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8074101
  Stats: 59 lines in 8 files changed: 29 ins; 20 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2046.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2046/head:pull/2046

PR: https://git.openjdk.java.net/jdk/pull/2046