RFR: 8323780: Serial: Remove unused _full_collections_completed

[Albert Mingkun Yang]

Trivial removing effectively dead code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/17440/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17440&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323780
  Stats: 24 lines in 2 files changed: 0 ins; 24 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17440.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17440/head:pull/17440

PR: https://git.openjdk.org/jdk/pull/17440