WL#7436: Deprecate and remove timed_mutexes system variable

The timed_mutexes system variable has been disconnected from any underlying InnoDB 
code, so it has no function.

Per Server PT decision 2013-11-27:

Deprecate timed_mutexes in 5.5/5.6 (throw a deprecation warning for attempts to 
set the variable), and remove it completely in 5.7.

User Documentation
==================

http://dev.mysql.com/doc/relnotes/mysql/5.5/en/news-5-5-39.html
http://dev.mysql.com/doc/relnotes/mysql/5.6/en/news-5-6-20.html
http://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-5.html