Multi-dataset fitting interface: removing spectra doesn't remove curves.

[mantid-roman]

Blocks #11929.

Another “bug” – removing spectra from the top of the list (or perhaps the middle) doesn’t remove the corresponding fit curves from the “list” so you get spectrum 2 displayed along with spectrum 1’s fit curve (and spectrum 1’s old difference). I’ve not yet checked what happens to the list of individual parameters, though stepping through the plots doesn’t update the values in the fit function pane, but it all sorts itself out when you hit “Fit” again.

[mantid-roman]

The issue was fixed in #13771.