fdo#75757: remove inheritance to std::map

from SfxItemPtrMap.

Change-Id: Id7e9667f9b918afaf92d9e71bf0e2c2e9c296474
Reviewed-on: https://gerrit.libreoffice.org/11696
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>