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

The option `usePlainDatagramSocketImpl` for the test `DatagramChannel/SendReceiveMaxSize.java` was not removed during `JDK-8253119 - Remove the legacy PlainSocketImpl and PlainDatagramSocketImpl implementation`. 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.