Drop Clear Linux support

Drop Clear Linux support

Due to lack of a maintainer, and deviating from established practices
almost universally (custom bootloader, custom package manager, ...),
it's not productive to keep support for Clear Linux in mkosi. We have
no idea if it works at all, no idea if something we do is going to
break it, and haven't received any feedback from Clear Linux users
for multiple years. Hence let's drop support for Clear Linux from
mkosi to reduce maintenance costs.