RFR: 8321120: Shenandoah: Remove ShenandoahElasticTLAB flag

[Aleksey Shipilev]

We have added `ShenandoahElasticTLAB` a long time ago, to provide the escape hatch if elastic TLABs would misbehave. We have been running with elastic TLABs for years without problems. Taking care of elastic/non-elastic TLAB allocs adds to technical debt. 

Additional testing:
  - [x] hotspot_gc_shenandoah

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Commit messages:
 - Remove stale comment as well
 - Fix

Changes: https://git.openjdk.org/jdk/pull/16907/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16907&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/jdk/pull/16907.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16907/head:pull/16907

PR: https://git.openjdk.org/jdk/pull/16907