Mini-toolbar button "remove" with the following sub-items (or just these sub-items for "delete" button):
"Remove from this category" for Category
"Unrated" for Rating
"Uncolored" for Color label
Perhaps also:
"Untag" in Tag, as subitem
Remove for Category, Rating, Colour label minitoolbar button
Moderators: helmut, XnTriq, xnview