CMake: Remove own FindLibXml2
CMake: Remove own FindLibXml2

Summary: The upstream (bundled with CMake) one is just fine.

Reviewers: #kde_telepathy, davidedmundson, apol, heikobecker

Reviewed By: apol, heikobecker

Tags: #kde_telepathy

Differential Revision: https://phabricator.kde.org/D24067

KDE/telepathy-logger-qt,KDE/telepathy-logger-qt,KDE/telepathy-logger-qt