Add DeleteCell to ScDocFunc, which is a variant of DeleteContents ...

for a single cell case.  This is equivalent of ScViewFunc::DeleteConents
with bSimple = true.

Change-Id: I5eccce21843f4cb21b29f96b0360617a180aebee
Have ScViewFunc::DeleteContents() to call ScDocFunc's.

If bSimple = true, call DeleteCell(), otherwise call DeleteContents().

Change-Id: Ic08d9501797b537430bef0b6f3d920c3b497104b