Remove dependency on xbmc ppa

[maayank]

1. The code uses nlohmann/json.hpp
2. It is provided by nlohmann-json3-dev
3. It was not available in the official Ubuntu 18 package repo and thus in the notes the PPA of Kodi (team-xbmc) is mentioned as dependency, e.g.

Azure-DCAP-Client (master)> grep -R team-xbmc
build/Phase2/install_prereqs.cmd
12:add-apt-repository ppa:team-xbmc/ppa -y

src/Linux/README.MD
9:* `sudo  add-apt-repository ppa:team-xbmc/ppa -y`

4. nlohmann-json3-dev is available in the official repo for Ubuntu 20 and up, e.g. https://packages.ubuntu.com/focal/nlohmann-json3-dev

Since dcap deprecated support for Ubuntu 18, the ppa is not needed. As it is best practice not to have unneeded PPAs, I suggest removing this from the docs.