Sheet: Further cleanup in Sheet class

With PR 21445 an unused function has been removed. But the underlying
class member isn't used anywhere else and can be removed as well.

src/Mod/Spreadsheet/App/Sheet.cpp

@@ -146,11 +146,6 @@ void Sheet::clearAll()
     cellErrors.clear();
     columnWidths.clear();
     rowHeights.clear();
-
-    for (auto& observer : observers) {
-        delete observer.second;
-    }
-    observers.clear();
 }
 
 // validate import/export parameters