RFR: 8378744: Obsolete NewSizeThreadIncrease flag

[Albert Mingkun Yang]

Obsolete the Serial GC only `NewSizeThreadIncrease` flag since it's difficult to determine what value to set.

Test: tier1-5

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Commit messages:
 - sgc-new-size-thread-increase

Changes: https://git.openjdk.org/jdk/pull/29930/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29930&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8378744
  Stats: 200 lines in 20 files changed: 1 ins; 198 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/29930.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29930/head:pull/29930

PR: https://git.openjdk.org/jdk/pull/29930