RFR: 8315450: G1: Remove redundant adjustment to GCDrainStackTargetSize

[Albert Mingkun Yang]

Simple removing redundant check & write to an global VM flag.

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Commit messages:
 - g1-remove-default

Changes: https://git.openjdk.org/jdk/pull/15510/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15510&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315450
  Stats: 5 lines in 1 file changed: 0 ins; 5 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15510.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15510/head:pull/15510

PR: https://git.openjdk.org/jdk/pull/15510