End an unassigned key with ended and remove no key of the chain (issue 1093)

ADR 0408 section 4 says no key of the chain is removed, a delete marker included. The Backend loses Remove and the Chain RemoveAssignment. An unassignment, and the end of a value only pulled in by another, write the value again with ended set, by compare-and-set; assigning it again writes a fresh standing value over it. A value with ended or a state is no assignment (Assignment.Stands) to the assignment lists, the joins, forgetting a module, seat holdings, the given values, the move and the assignment-changed event.

internal/chain/chain.go

@@ -182,20 +182,8 @@ type Chain struct {
 	Rollouts    Bucket[Rollout]
 	Assignments Bucket[Assignment]
 
-	rollouts    Backend
-	assignments Backend
-	holds       Holds
-}
-
-// RemoveAssignment ends an assignment's key with a marker, at the revision it was read at, only by a controller that holds
-// the lease. **The one removal the chain has**, for an unassignment in the delivery that moves the assignments: the
-// key's earlier values stay (history), and nothing is purged. The delivery after it keeps an unassigned key with an end
-// mark in its place and removes this (issue 1096). Builds and rollouts have none.
-func (c *Chain) RemoveAssignment(ctx context.Context, key string, revision uint64) error {
-	if err := c.Assignments.guard(ctx, key, nil); err != nil {
-		return err
-	}
-	return c.assignments.Remove(ctx, key, revision)
+	rollouts Backend
+	holds    Holds
 }
 
 // RolloutsOf is the rollouts bucket typed as the controller's plan, which carries more than the chain's Rollout names:
@@ -209,7 +197,6 @@ func New(builds, rollouts, assignments Backend, holds Holds) *Chain {
 		Rollouts:    Bucket[Rollout]{rollouts, holds},
 		Assignments: Bucket[Assignment]{assignments, holds},
 		rollouts:    rollouts,
-		assignments: assignments,
 		holds:       holds,
 	}
 }