RFR: 8303562: Remove obsolete comments in os::pd_attempt_reserve_memory_at

[Fei Yang]

Hi,
    Please review this trivial change removing some obsolete code comments in os::pd_attempt_reserve_memory_at.
    These comments became obsolete after JDK-8224871 which removes the retry loop to mmap random memory.

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Commit messages:
 - 8303562: Remove obsolete comments in os::pd_attempt_reserve_memory_at

Changes: https://git.openjdk.org/jdk/pull/12849/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12849&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303562
  Stats: 6 lines in 2 files changed: 0 ins; 6 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12849.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12849/head:pull/12849

PR: https://git.openjdk.org/jdk/pull/12849