RFR: 8298303: (fs) temporarily remove Path.getExtension

[Brian Burkhalter]

Remove `java.nio.file.Path::getExtension` from the JDK 20 mainline to allow its definition time to settle.

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Commit messages:
 - 8298303: (fs) temporarily remove Path.getExtension

Changes: https://git.openjdk.org/jdk/pull/11566/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11566&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298303
  Stats: 143 lines in 2 files changed: 0 ins; 142 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11566.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11566/head:pull/11566

PR: https://git.openjdk.org/jdk/pull/11566