MergeCell function did not remove overlapped mergecells

**Description**
Following is comments of MergeCell
```
// If you create a merged cell that overlaps with another existing merged cell,
// those merged cells that already exist will be removed.
...
		// Delete the merged cells of the overlapping area.
```
But actually, it didn't remove overlapped mergecells as it used to be.
I think it's about commit ` a88459`.