binary-search-tree: remove unused import

exercises/binary-search-tree/src/test/java/BinarySearchTreeTest.java

@@ -6,7 +6,6 @@ import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotNull;
 import org.junit.Ignore;
 import org.junit.Test;
-import org.junit.Before;
 
 public class BinarySearchTreeTest {