and drop DestroyMenuItem, all the implementations were just deleting the
pointer

Change-Id: I058817b963988b1e399279f61f45716a2cdba023
Reviewed-on: https://gerrit.libreoffice.org/55500Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>