RFR: 8301098: Remove dead code FileMapInfo::stop_sharing_and_unmap()

[Ioi Lam]

Please review this trivial patch. The function `FileMapInfo::stop_sharing_and_unmap()` is no longer being used so it should be removed.

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Commit messages:
 - 8301098: Remove dead code FileMapInfo::stop_sharing_and_unmap()

Changes: https://git.openjdk.org/jdk/pull/12205/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12205&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301098
  Stats: 26 lines in 2 files changed: 0 ins; 26 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12205.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12205/head:pull/12205

PR: https://git.openjdk.org/jdk/pull/12205