kunit: Remove the unused all_tests.config

This isn't used anywhere. While it's possible that people were manually
referencing it, the new default config (in default.config in the same
path) provides equivalent functionality.

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