RFR: 8301337: Remove unused os::_polling_page

[Robbin Ehn]

Hi, please consider removing the unused field _polling_page in os.

Built tested many platforms + t1.

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Commit messages:
 - 8301337: Remove unused os::_polling_page

Changes: https://git.openjdk.org/jdk/pull/12287/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12287&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301337
  Stats: 10 lines in 4 files changed: 0 ins; 10 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12287.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12287/head:pull/12287

PR: https://git.openjdk.org/jdk/pull/12287