Remove ftpd(8)

This was previously deprecated and is slated for removal in 15.0.
Users who still need ftpd(8) can install the ftp/freebsd-ftpd port.

Retain the ftp(d) PAM services since other FTP daemons use them.

Update /etc/inetd.conf to point to /usr/local.

Add ftpd to ObsoleteFiles, but do not list configuration files since
users may want to preserve these to use with the freebsd-ftpd port.

There is still some language in the manual referring to ftpd(8)
which is relevant to the port, which has been retained but updated
to reference the port.

MFC after: 3 days
Relnotes: yes
Reviewed by: cperciva
Differential Revision: https://reviews.freebsd.org/D52739

(cherry picked from commit 614c8750ce116865953916b8b5bab2467ee94187)