The only place is is used is ConvertAndPutItems.
Which only loops over EE_PARA* which ids.
But the only things which define slotid<->whichid mappings are XATTR*,
SDATTR*, SCHATTR*.
Which means that this code ConvertAndPutItems is dead, and that is the
last place using GetTrueSlotId.

Change-Id: I3037c25c2c15087572abacc9642970936824678d
Reviewed-on: https://gerrit.libreoffice.org/47377Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>