Remove Portsmon references

[dbaio]

Summary

Portsmon is broken since 2019, and it's been a year since we added a
note about it not working in the documentation.

D29717 - a4f1e085
D29715 - 2644b2cd

[dbaio]

We can also redirect its requests to another place, like:

https://ports.freebsd.org --> https://www.freebsd.org/ports/
http://portsmon.freebsd.org/ --> https://www.freebsd.org/ or https://www.freebsd.org/ports/