dhcp: drop dhcpcanon support
Summary
Drop support for the "dhcpcanon" DHCP client. It's unmantained, as the last code change was in 2018:
https://github.com/juga0/dhcpcanon/commits
There is no need to first deprecate it because it was still marked as "experimental" in NM. Also, it's not packaged by any recent distro, so we can assume that nobody will miss it.