RFR: 8329471: Remove GTK2 [v2]

[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 three additional commits since the last revision:

 - review comments
 - revert == to equals changes in favor of https://bugs.openjdk.org/browse/JDK-8337498
 - remove other leftover parts

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20378/files
  - new: https://git.openjdk.org/jdk/pull/20378/files/574358df..94d999fb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20378&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20378&range=00-01

  Stats: 82 lines in 12 files changed: 3 ins; 52 del; 27 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