RFR: 8253119: Remove the legacy PlainSocketImpl and PlainDatagramSocketImpl implementation

[Patrick Concannon]

Hi,

Could someone please review my changes for the removal of the legacy `PlainSocketImpl` and `PlainDatagramSocketImpl` implementations?

In JDK 13, JEP 353 provided a drop in replacement for the legacy `PlainSocketImpl` implementation. Since JDK 13, the `PlainSocketImpl` implementation was no longer used but included a mitigation mechanism to reduce compatibility risks in the form of a JDK-specific property `jdk.net.usePlainSocketImpl` allowing to switch back to the old implementation. 
Similarly, in JDK 15, JEP 373 provided a new implementation for `DatagramSocket` and `MulticastSocket`, with a JDK-specific property `jdk.net.usePlainDatagramSocketImpl` also allowing the user to switch back to the old implementation in case of compatibility issue.

As these implementations (and the mechanisms they use to enable them to mitigate compatibility issues) have been deemed no longer necessary, they now represent a maintenance burden. This patch looks at removing them from the JDK.

Kind regards,
Patrick

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Commit messages:
 - 8253119: Remove the legacy PlainSocketImpl and PlainDatagramSocketImpl implementation

Changes: https://git.openjdk.java.net/jdk/pull/4574/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4574&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253119
  Stats: 11046 lines in 78 files changed: 137 ins; 10836 del; 73 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4574.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4574/head:pull/4574

PR: https://git.openjdk.java.net/jdk/pull/4574