RFR: 8308588: Unnecessary synchronized on GTKStyle#ICONS_MAP can be removed [v2]

[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.

Abhishek Kumar has updated the pull request incrementally with one additional commit since the last revision:

  Use Collections.unmodifiableMap and remove blank line

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/20741/files
  - new: https://git.openjdk.org/jdk/pull/20741/files/5775df65..af1fdbdc

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20741&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20741&range=00-01

  Stats: 17 lines in 1 file changed: 1 ins; 1 del; 15 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