Removed unused import.

Author: gmessner

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

@@ -1,7 +1,6 @@
 package org.gitlab4j.api;
 
 import static org.junit.Assert.assertNotNull;
-import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assume.assumeTrue;