Make udev seriously mandatory

Summary:
73e13daef13 done just half work of ripping of HAVE_UDEV in c++ code,
this however didn't remove the UDEV_FOUND ifdefs from the cmake code, so
this UDEV_FOUND ifs in cmake code makes very less sense.

Test Plan: builds

Reviewers: broulik, Plasma

Reviewed By: broulik, Plasma

Subscribers: graesslin, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D1742