RFR: 8378379: Remove reference to obsolete jdk.net.usePlainSocketImpl property from SSLSocketReset test

[Jaikiran Pai]

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

`jdk.net.usePlainSocketImpl` was removed in https://bugs.openjdk.org/browse/JDK-8253119 and plays no role anymore in the JDK. This `test/jdk/sun/security/ssl/SSLSocketImpl/SSLSocketReset.java` test was introduced to reproduce a bug in the implementation of PlainSocketImpl, but doesn't rely on any internal socket implementation details, so I think it should be OK to retain this test and just remove the usage of the property. The property has been playing no role in this test, and the test has been functional, ever since the property was removed in  JDK-8253119.

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Commit messages:
 - 8378379: Remove reference to obsolete jdk.net.usePlainSocketImpl property from SSLSocketReset test

Changes: https://git.openjdk.org/jdk/pull/29872/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29872&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8378379
  Stats: 6 lines in 1 file changed: 0 ins; 5 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/29872.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29872/head:pull/29872

PR: https://git.openjdk.org/jdk/pull/29872