RFR: 8329471: Remove GTK2 [v5]

[Alexander Zvegintsev]

> 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()

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20378/files
  - new: https://git.openjdk.org/jdk/pull/20378/files/b7665a77..6b7e2c5c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20378&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20378&range=03-04

  Stats: 38 lines in 4 files changed: 3 ins; 12 del; 23 mod
  Patch: https://git.openjdk.org/jdk/pull/20378.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20378/head:pull/20378

PR: https://git.openjdk.org/jdk/pull/20378