RFR: 8344189: Cleanup code in sun.net.www.protocol.mailto.MailToURLConnection and sun.net.smtp after JEP 486 integration

[Eirik Bjørsnøs]

Please review this JEP486 cleanup PR in the `s.n.w.p.mailto.MailToURLConnection` and `sun.net.smtp.SmtpClient` classes.

* The `MailToURLConnection::getPermission` override with the related `permission` field is removed
* The SM property access check in `MailToURLConnection::getFromAddress` is removed
* The `SmtpClient` use of `GetPropertyAction` is replaced with plain `System::getProperty`

As a side note I have been programming Java since 1998 and I was not aware that this very limited (untested?) mechanism for sending email existed in the JDK. Unless it is in extensive use, perhaps we should consider deprecating it somehow. Superior alternatives exist.

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Commit messages:
 - Remove SecurityManager permission checking from SMTPClient
 - Remove SecurityManager permission checking from MailToURLConnection

Changes: https://git.openjdk.org/jdk/pull/22138/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22138&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344189
  Stats: 24 lines in 2 files changed: 0 ins; 20 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/22138.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22138/head:pull/22138

PR: https://git.openjdk.org/jdk/pull/22138