Remove `environment_timeout_mode` setting

The environment_timeout setting is a less-than-ideal way of dealing with environment purging, because it doesn't take into account the actual usage patterns for any given environment, and forces the user to make a call on whether to accept longer load times (timeout of 0), inconsistent state between JRubies (a timeout between 0 and unlimited), and high memory usage (unlimited timeout).

The environment_timeout_mode setting was added in PUP-8014, and this ticket is to remove the setting and change the default (and only) behavior so that puppet evicts environments that haven't been used for a certain amount of time.