RFR: 8254576: ZGC: Clean up timers in roots iterators

[Per Liden]

Some of the timers in roots iterators are no longer that interesting after JDK-8253180 (concurrent thread stack
scanning). I propose we remove them. This patch also adjusts the line break of a comment and moves a closure closer to
the function where it's used.

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Commit messages:
 - 8254576: ZGC: Clean up timers in roots iterators

Changes: https://git.openjdk.java.net/jdk/pull/605/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=605&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254576
  Stats: 38 lines in 2 files changed: 6 ins; 28 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/605.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/605/head:pull/605

PR: https://git.openjdk.java.net/jdk/pull/605