Change form feature also shows the current form in the list

When you change a catalogue item's form with "change form" feature, it also shows the current form the catalogue item already has. You can choose it, but it doesn't do anything.

See screenshot:

Start at the Change form feature and trace how its form-selection list is built. The list should omit the catalogue item's current form, while still allowing another form to be selected; check the relevant UI tests if the feature has them.

Written by the indexing model from the issue text.