RFR: 8253119: Remove the legacy PlainSocketImpl and PlainDatagramSocketImpl implementation [v4]

[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

Patrick Concannon has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:

 - 8253119: Added message to null check in java/net/DatagramSocket
 - Merge remote-tracking branch 'origin/master' into JDK-8253119
 - 8253119: Removed redundant comments and USE_PLAINDATAGRAMSOCKET from java/net/DatagramSocket
 - Merge remote-tracking branch 'origin/master' into JDK-8253119
 - 8253119: Remove the legacy PlainSocketImpl and PlainDatagramSocketImpl implementation

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4574/files
  - new: https://git.openjdk.java.net/jdk/pull/4574/files/49125e7e..36bcee31

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4574&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4574&range=02-03

  Stats: 7050 lines in 209 files changed: 1681 ins; 5063 del; 306 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