Consistency around SfxStringItem in svx/inc/sxonitm.hxx

...similar to what has been done for svx/sdtmfitm.hxx in
6a2ea81ca1622d2c2ad55bea8ddc28167fcc2794 "Remove unused ctors" and
68969cc61adecac481ae9656978ef952f435b310 "Consistency around SdrMetricItem."

Change-Id: I694c42a53cb9041b51ef876416f0f61963b89f86