RFR: 8303081: Serial: Remove unused VM_MarkSweep

[Albert Mingkun Yang]

Trivial removing dead code.

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

Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/12720/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12720&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303081
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12720.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12720/head:pull/12720

PR: https://git.openjdk.org/jdk/pull/12720