Remove unused code from tool/main.py

Removed unused threading/os code from tool/main.py and
tool/mocktool.py.

BUG=None

Review URL: https://codereview.chromium.org/563533002

git-svn-id: svn://svn.chromium.org/blink/trunk@181807 bbb929c8-8fbe-4397-9dbb-9b2b20218538