RFR: 8321120: Shenandoah: Remove ShenandoahElasticTLAB flag

[Y. Srinivas Ramakrishna]

On Thu, 30 Nov 2023 15:50:52 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> 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

Marked as reviewed by ysr (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/16907#pullrequestreview-1758364128