removeObserver method
Remove an observer that was previously registered with addObserver.
If addObserver was called more than once for observer,
removeObserver must be called the same number of times before observer
is removed.
Once removed observer is not called again.
If observer is not a registered observer of the cell, this method does
nothing.
Implementation
@override
void removeObserver(CellObserver observer) {
// Do nothing since the cell never has any observers
}