ErrorProne: fix sampler/UnusedMethod

sampler/src/main/java/atlantafx/sampler/page/showcase/musicplayer/ColorThief.java

@@ -16,6 +16,7 @@ import java.util.*;
 import static java.awt.image.BufferedImage.TYPE_3BYTE_BGR;
 import static java.awt.image.BufferedImage.TYPE_4BYTE_ABGR;
 
+@SuppressWarnings("unused")
 final class ColorThief {
 
     private static final int DEFAULT_QUALITY = 10;

sampler/src/main/java/atlantafx/sampler/theme/ThemeManager.java

@@ -223,6 +223,7 @@ public final class ThemeManager {
         customCSSRules.put(selector, rule);
     }
 
+    @SuppressWarnings("unused")
     private void removeCustomRule(String selector) {
         customCSSRules.remove(selector);
     }