RFR: 8329471: Remove GTK2 [v5]

[Magnus Ihse Bursie]

On Wed, 31 Jul 2024 19:17:05 GMT, Alexander Zvegintsev <azvegint at openjdk.org> wrote:

>> GTK2 support for Swing/AWT was deprecated for removal in JDK 21. 
>> 
>> It's being removed because no platform that supports the JDK needs it.
>> 
>> Manual and automated testing is green.
>
> Alexander Zvegintsev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   more removal of is3()

Build changes are trivially fine.

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Marked as reviewed by ihse (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/20378#pullrequestreview-2219324329