RFR: 8304833: (fc) Remove dead code in sun.nio.ch.FileChannelImpl::implCloseChannel

[Brian Burkhalter]

Remove `fdAccess.close(fd)` branch in `implCloseChannel` as it is unreachable because exactly one of `parent` and `closer` can be non-`null`.

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Commit messages:
 - 8304833: (fc) Remove dead code in sun.nio.ch.FileChannelImpl::implCloseChannel

Changes: https://git.openjdk.org/jdk/pull/13168/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13168&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304833
  Stats: 8 lines in 1 file changed: 2 ins; 3 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/13168.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13168/head:pull/13168

PR: https://git.openjdk.org/jdk/pull/13168