RFR: 8256664: Shenandoah: Cleanup after JDK-8212879

[Zhengyu Gu]

JDK-8212879 removed last serial weak roots, that means:

1) Concurrent GC no longer needs to cleanup any weak roots at pause
2) Root verifier no longer needs to distinguish concurrent/serial weak roots.

Test:
- [x] hotspot_gc_shenandoah

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Commit messages:
 - Update comment
 - Cleanup after JDK-8212879

Changes: https://git.openjdk.java.net/jdk/pull/1336/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1336&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256664
  Stats: 70 lines in 9 files changed: 9 ins; 37 del; 24 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1336.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1336/head:pull/1336

PR: https://git.openjdk.java.net/jdk/pull/1336