Removed unused import.

src/test/java/org/gitlab4j/api/TestCommitsApi.java

@@ -2,7 +2,6 @@ package org.gitlab4j.api;
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotNull;
-import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assume.assumeTrue;