Revert commit 0c220608, since this
apparently causes failure to open ODF files via SMB on Linux.

Instead of changing SfxMedium, adjust the ScExportTest: when saving
XLSX files interactively, the SfxFilter::lFormat is always 0, which
causes SfxObjectShell::SaveTo_Impl not to call
SfxMedium::GetOutputStorage() at all, working around the locking issue
on WNT.

Change-Id: I0ab8b4262475e4c8425eebb1413997457e5e9417