RFR: 8336879: Always true condition 'img != null' in GTKPainter.paintPopupMenuBackground

[Abhishek Kumar]

In GTKPainter.paintPopupMenuBackground method, `img != null` condition will always be true, because it's only checked after `img.getRGB` method is called and that means img can't be `null`.  So, the null check condition is removed. CI testing is ok.

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Commit messages:
 - remove null check

Changes: https://git.openjdk.org/jdk/pull/20308/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20308&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336879
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20308.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20308/head:pull/20308

PR: https://git.openjdk.org/jdk/pull/20308