This reverts commit edf2541.

Turns out lot of software (including Chromium) use bundled fontconfig
so we either need to wrap every one of those, or re-introduce the global
unversioned config. The latter is easier but weakens hermetic configs.

But perhaps those are not really worth the effort. Given that programs
linked against fc 2.12.6 on fc 2.13.92 system seem to at least display text,
even while printing tons of errors (as long as you generate fc cache manually),
and same thing the other way around, hopefully it will not be an issue in the future.

We give up on the hermetic configs in exchange for getting rid of lot of complexity.

The incompatibility does not seem to exist any more: programs linked against fc 2.12
on fc 2.14 system seem to at least display text, even while printing tons of errors
(as long as you generate fc cache manually), and same thing the other way around.
Hopefully it will not be an issue in the future.

The use of the argument was removed in 2016 but its definition was left behind.

NixOS@cd2948a