Remove non-actual puzzle (#462)

Author: dgarus

asto-core/src/main/java/com/artipie/asto/SubStorage.java

@@ -19,10 +19,6 @@
  * It decorates origin storage and proxies all calls by appending prefix key.
  * </p>
  * @since 0.21
- * @todo #139:30min Implement prefixed transaction support.
- *  Transaction extends storage, so it's needed to implement all transactions method
- *  with prefixed key too. Also, create unit tests for sub storage to verify that
- *  all methods uses prefixed keys.
  */
 public final class SubStorage implements Storage {