Remove extension method MorphicToolBuilder>>#buildPluggableMultiSelectionList:

[LinqLover]

• The extension is not classified properly as an override (leading to dirty Monticello working copies)
• The override is no longer necessary, getIconSelector: is already set by the default Squeak implementation since 2017
• The override is not up to date with the current implementation in Squeak, leading to missing selectors such as getHelpSelector

[LinqLover]

I have updated the override. Should be merged into trunk