Fix Mantis issue#103 mysql_addons.c depends on THD internals - please remove it

Use MySQL's thd_get_thread_id() instead of our own ib_thd_get_thread_id()
since MySQL implemented the former, see http://bugs.mysql.com/30930.

Approved by:	Marko (https://svn.innodb.com/rb/r/40, rb://40)