Revert "tzsetup: symlink /etc/localtime instead of copying"

This failed when used with tzsetup's -C option (for example, when using
etcupdate -D to update a jail from the host). Revert the stable/14 MFC
for now; will be reapplied after being fixed in main..

This reverts commit fc43a1b6842afa806dfd7ba48de5bece63d04456.
This reverts commit 87f7f0389f8b7bf30ef12df5c0d337cb2789883e.