Remove NX support

Summary:
Although NX still exists, and nomachine.com offers downloads for
Linux, the NX plugin for krdc was never updated since KDE 4.2.
Let's call 8 years with no development a "unused, remove" kind of feature.

Test Plan:
The top-level CMakeLists.txt had the check for NX commented out,
and the sub-CMakeLists.txt checked for a _FOUND that couldn't
possibly be set.

Reviewers: uwolfer

Subscribers: aacid

Differential Revision: https://phabricator.kde.org/D23453