RFR: 8371643: Remove ThreadLocalAllocBuffer::_reserve_for_allocation_prefetch

[Albert Mingkun Yang]

Trivial removing obsoleted code for unsupported arch.

Test: tier1

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Commit messages:
 - remove-tlab-reserve

Changes: https://git.openjdk.org/jdk/pull/28240/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28240&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371643
  Stats: 29 lines in 3 files changed: 0 ins; 28 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28240.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28240/head:pull/28240

PR: https://git.openjdk.org/jdk/pull/28240