Consistency around SdrMetricItem in svx/sxelditm.hxx

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

(SdrEdgeLine1DeltaItem::Clone had been recently---and in retrospect needlessly
---introduced in 596ec6f52fe3a8d56d5c20df568633cfbcadc74d "Missing
SdrEdgeLine1DeltaItem::Clone override" before
3efa3ad7b6de78c291f49858e50e6bedbd6a2173 "SdrEdgreLine*DeltaItem::Put/QueryValue
are same as underlying" revealed that SdrEdgeLine1DeltaItem was a needless
derivation of SdrMetricItem.)

Change-Id: I154d32d46a7c01c792aa4e812cf576d4d749e0c9