Remove determining time zone via systemsetup on macOS

Remove determining time zone via systemsetup on macOS

According to https://truesecdev.wordpress.com/2015/04/09/hidden-backdoor-api-to-root-privileges-in-apple-os-x/comment-page-1/
the `systemsetup` command has required superuser privileges since
macOS 10.8.5 (which has been EOL for over 6 years at the time of writing).
We shouldn't expect to use a codepath that requires a helper tool
that requires su in any regular use; IOW, _if_ a Babel user had ever
reached this path without having been superuser, it would have failed anyway
on any currently supported version of macOS.

Closes #895 (supersedes it).