locale-setup: default to C.UTF-8

locale-setup: default to C.UTF-8

Most distributions already were shipping a C.UTF-8 locale and even Fedora
now supports the C.UTF-8 locale, and there's clear indication that this
is going upstream too. Hence, let's default to it now too, if nothing
else is set.

Note that this is only a fallback if noting else is set, and since
distros generally configure a default for this behaviour shouldn't
really change in installed systems.

On new systems this makes vconsole.conf redundant.