Remove the self-imposed 2 minute limit from hang_analyzer.py

As part of SERVER-14181, the hang_analyzer.py script set a timer to cause itself to exit after 2 minutes. With the changes from EVG-580, this is no longer necessary because Evergreen now sets a default time limit for the "timeout" command of 15 minutes. The time limit is also configurable via a callback_timeout_secs property in the etc/evergreen.yml project configuration.

It is desirable to allow the hang_analyzer.py script to run longer than 2 minutes in order to ensure it has ample time to dump thread stacks of any MongoDB processes still running when the task times out. Note: Until SERVER-27158 is implemented, there may be many MongoDB processes still running when the task times out.