[findBugs] Remove nonsense call

Result of c.containsAll(c)  is always true for any collection c.

Change-Id: I853cbb44bdc03e2b54229bb9c36ae42e02dcc4d9
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/revwalk/RevFlagSetTest.java

@@ -103,7 +103,6 @@ public class RevFlagSetTest extends RevWalkTestCase {
 		assertTrue(set1.add(flag1));
 		assertTrue(set1.add(flag2));
 
-		assertTrue(set1.containsAll(set1));
 		assertTrue(set1.containsAll(Arrays
 				.asList(new RevFlag[] { flag1, flag2 })));