RFR (XS) 8175267: [s390] cleanup stub code "handler_for_unsafe_access"

Hi all, May I kindly request reviews for my very small cleanup change? Further down the road, I would need a sponsor, too. Bug: https://bugs.openjdk.java.net/browse/JDK-8175267 Webrev: http://cr.openjdk.java.net/~lucy/webrevs/8175267/ Description: The "handler_for_unsafe_access” is a relic from former times, having survived only on s390x. The stub is generated, but never used (called). This change removes everything related. Thanks, Lutz