This reverts commit 2757ee9f.

Besides causing a performance regression, I now notice that
there is code in SW that relies on iterating over two different
SfxItemSet's in parallel, and assumes that missing items are
returned as nullptr, which is not the case for my std::map based
change.

Change-Id: I2b1110350fe4c4b74e5508558e9661ef1e1a103e