Do not throw ObjectDisposedException in ExtendedCommandHandler.Dispose()

# The problem

https://github.com/dotnet/iot/blob/80d67e0176fb49820adab393711cc5db1ecdd0c2/src/devices/Arduino/ArduinoBoard.cs#L701

# Solution

Remove `if(_isDisposed) throw new ObjectDisposedException();` somewhere.

# Alternatives

* `public bool IsDisposed { get; private set; }`

###### maybe it doesn't throw anything at all and try/catch is reduant here
###### ~~remove `api-suggestion` label~~