[FIX] doc: Kanban view: remove doc of dropped feature
[FIX] doc: Kanban view: remove doc of dropped feature

Before 9.0, it was possible to display aggregate values in grouped
Kanban view (like sum, average...). The aggregate was displayed in
the header of the column (see for example in 8.0 in Project >
Tasks).

This feature has been dropped in 9.0 because it didn't work
correctly, but the documentation hasn't been updated accordingly.

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