Delete BasicSpanTest.cs (#15860)
Delete BasicSpanTest.cs (#15860)

It is redundant with CoreFX Span tests now
mmitche/coreclr,ruben-ayrapetyan/coreclr,mmitche/coreclr,poizan42/coreclr,krk/coreclr,wtgodbe/coreclr,wtgodbe/coreclr,yizhang82/coreclr,JosephTremoulet/coreclr,cshung/coreclr,poizan42/coreclr,poizan42/coreclr,poizan42/coreclr,cshung/coreclr,JosephTremoulet/coreclr,mmitche/coreclr,wtgodbe/coreclr,mmitche/coreclr,JosephTremoulet/coreclr,cshung/coreclr,ruben-ayrapetyan/coreclr,ruben-ayrapetyan/coreclr,yizhang82/coreclr,krk/coreclr,cshung/coreclr,JosephTremoulet/coreclr,krk/coreclr,cshung/coreclr,JosephTremoulet/coreclr,yizhang82/coreclr,wtgodbe/coreclr,mmitche/coreclr,krk/coreclr,yizhang82/coreclr,yizhang82/coreclr,wtgodbe/coreclr,mmitche/coreclr,ruben-ayrapetyan/coreclr,krk/coreclr,JosephTremoulet/coreclr,cshung/coreclr,krk/coreclr,yizhang82/coreclr,ruben-ayrapetyan/coreclr,ruben-ayrapetyan/coreclr,poizan42/coreclr,wtgodbe/coreclr,poizan42/coreclr