RFR: JDK-8344057 : Remove doPrivileged calls from unix platform sources in the java.desktop module

[Harshitha Onkar]

Post JEP-486 (Permanently Disable the Security Manager) cleanup.
Calls to java.security.AccessController.doPrivileged are obsolete thus removed in this PR.

This PR addresses removal of AccessController.doPrivileged() calls from unix-platform files in the java.desktop module. Any SM related imports that are no longer needed are removed.

This PR is limited to removing doPrivileged() calls and excludes any refactoring, reformatting, or other clean up that is out-of-scope for this fix.

PS: I have explicitly add comments to the changes where a more watchful review is required.

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Commit messages:
 - removed unused import
 - extra braces added
 - doPrivileged() removal for unix files

Changes: https://git.openjdk.org/jdk/pull/22218/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22218&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344057
  Stats: 540 lines in 25 files changed: 45 ins; 283 del; 212 mod
  Patch: https://git.openjdk.org/jdk/pull/22218.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22218/head:pull/22218

PR: https://git.openjdk.org/jdk/pull/22218