Remove use of evergreen_client library in favor of evergreen.py in burn_in_tests

The burn_in_tests script calls out to evergreen to get test history and other information about the task running. We recently build a python evergreen client and added it to the burn_in_tests buildscript. We should be consistent and use evergreen.py to access the evergreen api everywhere in the burn_in_tests script.

As a mongo engineer
I want burn_in_tests to use a common evergreen client library
so that I don't have to maintain code to connect to evergreen.

AC

• The burn_in_tests script does not use the evergreen client in buildscripts. 

Related ticket: https://jira.mongodb.org/browse/SERVER-40893