Unassign creators from campaign

Sets or replaces the scheduled end of the current campaign assignment for one or more creators. All creators must share the same campaign. endBasis prev, current or next end after a full billing period; today ends the assignment at the end of today and custom on the endDate named, any day. An end inside a billing period makes that period the assignment's final one, cut to the end day, and due the day after like any other (pay it, or cancel it if the creator was paid another way). An end on or before the assignment start clears it entirely; removeCanceledWindows consents to deleting canceled payout windows overlapping a cleared assignment (paid windows always block). Once a cut final period is paid or canceled the end cannot move anymore.