For some reasons there are [failures](https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8759809744202981089/+/u/run_test.dart_for_framework_tests_shard_and_subshard_misc/test_stdout) in leak tracking bots, that do not relate to leak tracking. 

There are two differences between leak tracking bots and 'normal' windows framework bots: 

1. The flag "LEAK_TRACKING": "true". First failures happened before consumption of the flag "LEAK_TRACKING": "true" by testWidgets. Hypothesis: the failures may be caused by mechanisms that sets the env variable. This mechanism is used only by leak tracking bots.
2. bringup: true. 

This PR removes the leak tracking env variable to isolate the first difference and verify if the bots will start passing.