RFR: 8326626: GenShen: Remove dead code associated with non-elastic TLABSRemove dead inelastic tlab code

[Kelvin Nilsen]

Remove extraneous remnant of code that is no longer needed because all LABs are not elastic.

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Commit messages:
 - Remove dead inelastic plab code
 - Revert "Remove dead code for inelastic plabs"
 - Remove dead code for inelastic plabs

Changes: https://git.openjdk.org/shenandoah/pull/400/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=400&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326626
  Stats: 8 lines in 1 file changed: 0 ins; 8 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/400.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/400/head:pull/400

PR: https://git.openjdk.org/shenandoah/pull/400