Fix of tdf#41996 is a workaround of kde4-specific condition and
has been since disabled on OS X (as tdf#51023). kde5 comes with
a new implementation of DnD and slide sorter in Impress now
exhibits exactly the same symptoms as on OS X in tdf#51023

Assuming that in 6.2 kde5 doesn't need this workaround anymore and
remaining platforms -- gtk[23] and Win -- never had issues with DnD
in slide sorter, keep it active only for kde4 vclplug.

We can't kill it with fire in its entirety, because kde4 has been
obsoleted only post-6.2

Change-Id: I1fd8bb9e5142effffbd43f3a8bead417832c808c
Reviewed-on: https://gerrit.libreoffice.org/73403
Tested-by: Jenkins
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>