tests: Remove Arch Linux SUPPORTED locale missing issue
Since glibc 2.36-4, Arch Linux started to include the
/usr/share/i18n/SUPPORTED
file.
https://github.com/archlinux/svntogit-packages/commit/3c537bee8d7921bc0af775fdb4d69124026532ac
For this reason we don't need to special case Arch Linux anymore in our tests.