RFR: 8257077: ZGC: Remove ZWorkers::run_serial()

[Per Liden]

`ZWorkers::run_serial()` is no longer used and can be removed.

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Commit messages:
 - 8257077: ZGC: Remove ZWorkers::run_serial()

Changes: https://git.openjdk.java.net/jdk/pull/1432/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1432&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257077
  Stats: 5 lines in 2 files changed: 0 ins; 5 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1432.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1432/head:pull/1432

PR: https://git.openjdk.java.net/jdk/pull/1432