RFR: 8308588: Unnecessary synchronized on GTKStyle#ICONS_MAP can be removed

[Abhishek Kumar]

`static final Map<String,GTKStockIcon> ICONS_MAP` is modified only in `static` block. Then [com.sun.java.swing.plaf.gtk.GTKStyle#get](https://github.com/kumarabhi006/jdk/blob/73f7a5f15dbba54a98f3916ff1190520ac07874d/src/java.desktop/share/classes/com/sun/java/swing/plaf/gtk/GTKStyle.java#L892) method read from it within `synchronized (ICONS_MAP)` block. As this map is read-only and all its content is initialized in static block we can safely remove synchronized from get call.

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Commit messages:
 - Remove Synchronized block

Changes: https://git.openjdk.org/jdk/pull/20741/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20741&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308588
  Stats: 4 lines in 1 file changed: 0 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20741.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20741/head:pull/20741

PR: https://git.openjdk.org/jdk/pull/20741