Deduplicate code

Change-Id: Ic11e481688401559c2e26cffd625d3b6e3bf2f03
Reviewed-on: https://gerrit.libreoffice.org/57219
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>

include/svl/itemset.hxx

@@ -158,11 +158,7 @@ public:
     }
     template<class T> const T* GetItem( TypedWhichId<T> nWhich, bool bSearchInParent = true ) const
     {
-        const SfxPoolItem* pItem = GetItem(sal_uInt16(nWhich), bSearchInParent);
-        const T* pCastedItem = dynamic_cast<const T*>(pItem);
-
-        assert(!pItem || pCastedItem); // if it exists, must have the correct type
-        return pCastedItem;
+        return GetItem<T>(sal_uInt16(nWhich), bSearchInParent);
     }