RFR: 8254562: ZGC: Remove ZMarkRootsTask [v3]

[Stefan Karlsson]

> After introducing concurrent stack scanning, we don't need to mark through any roots during the mark start pause.
> Remove the code.
> Note that the old code passed in `false` to `_roots(false /* visit_jvmti_weak_export */)`. This has the effect that no
> roots are visited by the iterator: void ZRootsIterator::oops_do(ZRootsIteratorClosure* cl) {
>   ZStatTimer timer(ZSubPhasePauseRoots);
>   if (_visit_jvmti_weak_export) {
>     _jvmti_weak_export.oops_do(cl);
>   }
> }

Stefan Karlsson has updated the pull request incrementally with one additional commit since the last revision:

  Review 1

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/601/files
  - new: https://git.openjdk.java.net/jdk/pull/601/files/12274fc0..6f053cf4

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=601&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=601&range=01-02

  Stats: 26 lines in 3 files changed: 24 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/601.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/601/head:pull/601

PR: https://git.openjdk.java.net/jdk/pull/601