Unsets XDG_RUNTIME_DIR in processes run with kdesu

Summary:
If we keep a XDG_RUNTIME_DIR value from another user then all kind of
breakages might happen. For example, if user foo runs
"kdesu -u bar dolphin", then when dolphin tries to get
QStandardPaths::writableLocation(RuntimeLocation), it returns an empty string
(because XDG_RUNTIME_DIR is not correct). which makes kio try
to write a socket to access kio slaves in / . Which, for most users
results in applications not being able to use kio (so no usable dolphin)
and for root, it means sockets were created in / .

Look at https://bugzilla.opensuse.org/show_bug.cgi?id=1009510 and
https://phabricator.kde.org/D3317 for more information

Reviewers: dfaure

Reviewed By: dfaure

Differential Revision: https://phabricator.kde.org/D3331