RFR: 8363229: Parallel: Remove develop flag GCExpandToAllocateDelayMillis

[Albert Mingkun Yang]

Simple removing a develop flag that is rarely (or even never) used.

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Commit messages:
 - pgc-remove-dev-flag

Changes: https://git.openjdk.org/jdk/pull/26425/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26425&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8363229
  Stats: 6 lines in 2 files changed: 0 ins; 6 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26425.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26425/head:pull/26425

PR: https://git.openjdk.org/jdk/pull/26425