RFR: 8344112: Remove code to support security manager execution mode from DatagramChannel implementation

[Alan Bateman]

Remove code required for the now defunct SecurityManager execution mode from DatagramChannelImpl. Dropping this execution mode means that untrustedReceive can be removed. Additionally, blockingReceive (used to support the adaptor) no longer needs a loop to drop datagrams that the SecurityManager rejects.

Once this change is in then it allow reverting some changes introduced to avoid pinning in the adaptor's receive method, a consequence of have both JEP 486 and JEP 491 integrated.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/22072/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22072&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344112
  Stats: 206 lines in 2 files changed: 4 ins; 168 del; 34 mod
  Patch: https://git.openjdk.org/jdk/pull/22072.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22072/head:pull/22072

PR: https://git.openjdk.org/jdk/pull/22072