RFR: 8346603: Cleanup Metacity.ThemeGetter class

[Andrey Turbanov]

There is opportunity to remove redundant nested class `Metacity.ThemeGetter`. It was introduced in the SecurityManager era to handle privileged call. Now it's not the case and we can remove it.
It simplifies and streamlines logic.

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Commit messages:
 - [PATCH] Cleanup Metacity.ThemeGetter

Changes: https://git.openjdk.org/jdk/pull/22292/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22292&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8346603
  Stats: 148 lines in 1 file changed: 45 ins; 67 del; 36 mod
  Patch: https://git.openjdk.org/jdk/pull/22292.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22292/head:pull/22292

PR: https://git.openjdk.org/jdk/pull/22292