Don't add plymouth to mkinicpio hooks as it's not used on the Pinephone Pro anymore
In the beginning we used Plymouth on the Pinephone Pro, but this is no longer the case.
So remove the section in the post-install to add the plymouth hook.
It is no longer found and mkinitcpio
complains about it, stating the image might not be complete.
Signed-off-by: Dan Johansen strit@manjaro.org