erase the correct element in ScColumn::Delete(), not the last one
erase the correct element in ScColumn::Delete(), not the last one

This fixes the dreaded crash in sc_unoapi test.
Thanks to Kohei for pointing in the right direction.

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core