RFR: 8348241: ZGC: Unnecessarily reinitialize ZFragmentationLimit's default value

[Zhengyu Gu]

Please review this trivial change to remove unnecessary reinitializing  ZFragmentationLimit's default value. Looks like a leftover from [JDK-8341692](https://bugs.openjdk.org/browse/JDK-8341692)

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Commit messages:
 - 8348241: ZGC: Unnecessarily reinitialize ZFragmentationLimit's default value

Changes: https://git.openjdk.org/jdk/pull/23225/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23225&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348241
  Stats: 4 lines in 1 file changed: 0 ins; 4 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/23225.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23225/head:pull/23225

PR: https://git.openjdk.org/jdk/pull/23225