gnomeos-contents: Don't symlink /tmp to /sysroot/tmp

Since tmpfs will be mounted on /tmp, we don't need to sysroot to
a permanent location /sysroot/tmp. Such a symlink will confuse
systemd when it tries to mount tmpfs on /tmp, so just make /tmp
an empty directory.

https://bugzilla.gnome.org/show_bug.cgi?id=732517