UIServiceTest: remove no-longer-needed override

Author: ctrueden

src/test/java/org/scijava/ui/UIServiceTest.java

@@ -71,11 +71,6 @@ public void testHeadlessUI() {
 
 	private static final class MockUserInterface extends AbstractUserInterface {
 
-		@Override
-		public SystemClipboard getSystemClipboard() {
-			return null;
-		}
-
 		@Override
 		public DisplayWindow createDisplayWindow(final Display<?> display) {
 			return null;