removing my.cnf parameter 'mysql_thread_concurrency'

- parameter is useless on Linux systems
- paramter only useful on Solaris version < 9
- removed on MySQL 5.6.1 (http://bugs.mysql.com/bug.php?id=55001)

http://www.percona.com/blog/2012/06/04/thread_concurrency-doesnt-do-what-you-expect/