RFR: 8254562: ZGC: Remove ZMarkRootsTask

[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);
  }
}

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

Commit messages:
 - 8254562: ZGC: Remove ZMarkRootsTask

Changes: https://git.openjdk.java.net/jdk/pull/601/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=601&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254562
  Stats: 38 lines in 1 file changed: 0 ins; 38 del; 0 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