RFR: 8256727: ZGC: Clean out dead code after JDK-8212879

[Stefan Karlsson]

There are some unused root processing functions after the JVMTI Tag Map was made concurrent. Clean them out.

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Commit messages:
 - 8256727: ZGC: Clean out dead code after JDK-8212879

Changes: https://git.openjdk.java.net/jdk/pull/1340/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1340&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256727
  Stats: 70 lines in 9 files changed: 2 ins; 67 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1340.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1340/head:pull/1340

PR: https://git.openjdk.java.net/jdk/pull/1340