RFR: 8302127: Remove unused arg in write_ref_field_post

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/12489/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12489&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302127
  Stats: 8 lines in 6 files changed: 0 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/12489.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12489/head:pull/12489

PR: https://git.openjdk.org/jdk/pull/12489