RFR: 8276634: Remove `usePlainDatagramSocketImpl` option from the test DatagramChannel/SendReceiveMaxSize.java

[Patrick Concannon]

Hi,

Could someone please review my changes for removing `usePlainDatagramSocketImpl` options from the test DatagramChannel/SendReceiveMaxSize.java? 

The option `usePlainDatagramSocketImpl` was not removed during [JDK-8253119 - Remove the legacy PlainSocketImpl and PlainDatagramSocketImpl implementation](https://bugs.openjdk.java.net/browse/JDK-8253119). As these implementations have now been removed from the JDK, tests no longer need to check against it.

The test `test/jdk/java/nio/channels/DatagramChannel/SendReceiveMaxSize.java` as well as `test/jdk/java/nio/channels/DatagramChannel/ChangingAddress.java` still have this option as its removal was overlooked in the original issue. It can now be removed from both tests. 


Kind regards,
Patrick

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Commit messages:
 - 8276634: Remove `usePlainDatagramSocketImpl` option from the test DatagramChannel/SendReceiveMaxSize.java

Changes: https://git.openjdk.java.net/jdk/pull/6252/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6252&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276634
  Stats: 3 lines in 2 files changed: 0 ins; 2 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6252.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6252/head:pull/6252

PR: https://git.openjdk.java.net/jdk/pull/6252