<AWT Dev> RFR: JDK-8273387: remove some unreferenced gtk-related functions

[Matthias Baesken]

Please review this small change.
Looks like there are a few functions, like gtk2_paint_diamond, that are unreferenced and can be removed.

Thanks, Matthias

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Commit messages:
 - JDK-8273387

Changes: https://git.openjdk.java.net/jdk/pull/5384/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5384&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273387
  Stats: 63 lines in 2 files changed: 0 ins; 61 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5384.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5384/head:pull/5384

PR: https://git.openjdk.java.net/jdk/pull/5384