This reverts most code, but not the implemented functionality of
commit 2fba5cb90e30f2bfab07782c9fa3b99275e769da.
Requiring a config file change to still be able to install old
distributions would make a bump of the major version necessary
according to Semantic Versioning. Let's avoid that:
The list of distributions with the old path scheme is finite and will
never change again. Hence it's ok and especially backwards compatible
to hardcode this list in hooks/debian/20-setup-apt.
It's also less and more obvious (but not necessarily easier to read)
code. But it's all in one place and not scattered over three files.
And it's just one line instead like a dozen or two dozens.
Closes: #972749 (kinda again)
eab2940bf6