ftpd(8): do not refer to now unused libxo(3)

In 2018, the commit r328100 (0fdf7fa846b1a1b1679e86812a1b08b8cb623604)
removed libxo(3) support from ls(1), so ftpd has no reasons to link
with libxo since then.

ls(1) does not depend on libxo in both of stable/12 and stable/13.

MFC after: 2 weeks