kunit: Remove duplicated defconfig creation

'--defconfig' option is handled by the 'main() of the 'kunit.py' but
again handled in following 'run_tests()'.  This commit removes this
duplicated handling of the option in the 'run_tests()'.

Signed-off-by: SeongJae Park <sjpark@amazon.de>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Tested-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>