Consistency around SdrMetricItem in svx/sdsxyitm.hxx

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

Change-Id: I3734cb14b6ed8f556af85b234968682a55ac8a4c