prevent KDE/Qt from interfering with the session manager

I occassionally get lockups in IceProcessMessages() called from QtCore,
I'm actually not exactly sure why, as theoretically two connections
from one app shouldn't be a problem, but since LO does its own
session handling, there's no need to the KDE/Qt code to be involved,
so prevent it from connecting to the session manager altogether.

Change-Id: Iebe20d4cb5403e5fea8bd5d8c1f69b62d1c2907b
(cherry picked from commit 71f2aff7a56cef4e133abad3c2e447c76c5ee1fe)
Reviewed-on: https://gerrit.libreoffice.org/9079
Reviewed-by: Maxim Monastirsky <momonasmon@gmail.com>
Tested-by: Maxim Monastirsky <momonasmon@gmail.com>