kunit: tool: remove unused imports and variables

We don't run a linter regularly over kunit.py code (the default settings
on most don't like kernel style, e.g. tabs) so some of these imports
didn't get removed when they stopped being used.

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>