MINOR: Remove release script (#3144)

Including the mention in the `LICENSE` and the `README.md` that
still refers to Jira.

Author: Fokko

LICENSE

@@ -197,16 +197,6 @@ License: Apache License Version 2.0 http://www.apache.org/licenses/LICENSE-2.0
 
 --------------------------------------------------------------------------------
 
-This product includes code from Apache Spark.
-
-* dev/merge_parquet_pr.py is based on Spark's dev/merge_spark_pr.py
-
-Copyright: 2014 The Apache Software Foundation.
-Home page: https://spark.apache.org/
-License: http://www.apache.org/licenses/LICENSE-2.0
-
---------------------------------------------------------------------------------
-
 This product includes code from Twitter's ElephantBird project.
 
 * parquet-hadoop's UnmaterializableRecordCounter.java includes code from