RFR: JDK-8269651: ZGC: Optimize away gc locker in mark start

[Hamlin Li]

Currently, ZGC involves gc locker in mark-start and relocate-start.
It seems gc locker in mark-start is not necessary anymore.
This is to remove this uncessary(??) gc locker before further optimization is discussed.

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Commit messages:
 - remove unnecessary gc locker at mark start

Changes: https://git.openjdk.java.net/jdk/pull/4638/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4638&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269651
  Stats: 4 lines in 1 file changed: 0 ins; 4 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4638.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4638/head:pull/4638

PR: https://git.openjdk.java.net/jdk/pull/4638