mkarchroot: stop adding C.UTF-8 to locale.gen
Adding such an entry is problematic as it results in locale -a showing:
C
C.UTF-8
C.utf8
POSIX
(other locales)
In the above, C.UTF-8 is built-in whereas C.utf8 comes from locale.gen.
The duplicate locale has the potential to expose undesired behavior in upstream software: https://github.com/sudo-project/sudo/issues/241