[jdk21u-dev] RFR: 8321120: Shenandoah: Remove ShenandoahElasticTLAB flag

[Aleksey Shipilev]

Clean backport to improve Shenandoah maintainability. 

Additional testing:
 - [x] macos-aarch64-server-fastdebug, `hotspot_gc_shenandoah`

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Commit messages:
 - Backport 93b9235f19dab2a0c08ba8a1afcc82ee71be4c0b

Changes: https://git.openjdk.org/jdk21u-dev/pull/177/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=177&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321120
  Stats: 92 lines in 5 files changed: 0 ins; 87 del; 5 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/177.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/177/head:pull/177

PR: https://git.openjdk.org/jdk21u-dev/pull/177