tools: ynl: don't regen on every make

As far as I can tell the normal Makefile dependency tracking
works, generated files get re-generated if the YAML was updated.
Let make do its job, don't force the re-generation.
make hardclean can be used to force regeneration.

Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/20231003153416.2479808-3-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>