Documentation: kunit: drop obsolete note about uml_abort for coverage

Commit b6d5799b ("kunit: Add 'kunit_shutdown' option") changes KUnit
to call kernel_halt() by default when done testing.

This fixes the issue with not having .gcda files due to not calling
atexit() handlers, and therefore we can stop recommending people
manually tweak UML code.

The need to use older versions of GCC (<=6) remains however, due to
linktime issues, same as before. Note: There also might still be issues
with .gcda files as well in newer versions.

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