removing unused assignment operators for SwDocDisplayItem,
SwElemItem and SwShadowCursorItem, which are implicitly deleted now.

Change-Id: I25200c3c61b77c5dedf7cf9de3edabc9d1e68730
Reviewed-on: https://gerrit.libreoffice.org/44928Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>