fix: remove unused createLines() in BatchingDescriptorComposerTest (#549)

Author: miraleung

src/test/java/com/google/api/generator/gapic/composer/BatchingDescriptorComposerTest.java

@@ -185,9 +185,4 @@ private static Method findMethod(Service service, String methodName) {
     }
     return null;
   }
-
-  private static String createLines(String... lines) {
-    // Cast to get rid of warnings.
-    return String.format(new String(new char[lines.length]).replace("\0", "%s"), (Object[]) lines);
-  }
 }