RFR: 8378378: Remove references to obsolete jdk.net.usePlainDatagramSocketImpl property from tests

[Jaikiran Pai]

Can I please get a review of this test-only change which removes reference to the obsolete `jdk.net.usePlainDatagramSocketImpl` property?

This property and the related fallback support was removed in https://bugs.openjdk.org/browse/JDK-8253119. The property plays no role anymore. The tests continue to pass after this change.

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Commit messages:
 - remove references to obsolete jdk.net.usePlainDatagramSocketImpl property

Changes: https://git.openjdk.org/jdk/pull/29871/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29871&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8378378
  Stats: 79 lines in 3 files changed: 0 ins; 50 del; 29 mod
  Patch: https://git.openjdk.org/jdk/pull/29871.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29871/head:pull/29871

PR: https://git.openjdk.org/jdk/pull/29871