Qt4 is on it's way out, according to
https://github.com/NixOS/nixpkgs/pull/174634
Barco's ClickShare driver/client requires Qt4;
an update isn't in sight anywhere.
To prepare for the removal of Qt4,
the commit at hand removes the
ClickShare package and its NixOS module.
The release notes are appended with a hint about the
removal and some alternatives that might help users
that are still in need of the driver/client functionality.
