RFR: 8344184: Remove sun.net.ResourceManager after JEP 486 integration

[Daniel Fuchs]

Now that JEP 486 has been integrated the sun.net.ResourceManager class has no purpose and can be removed.

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Commit messages:
 - 8344184: Remove sun.net.ResourceManager after JEP 486 integration

Changes: https://git.openjdk.org/jdk/pull/22394/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22394&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344184
  Stats: 109 lines in 3 files changed: 0 ins; 102 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/22394.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22394/head:pull/22394

PR: https://git.openjdk.org/jdk/pull/22394