Removes the deprecated DeadLetterChannel in ChannelableStatus (#6722)

Signed-off-by: Vishal Choudhary <sendtovishalchoudhary@gmail.com>

Fixes #6720 

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## Proposed Changes

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- 🎁 Add new feature
- 🐛 Fix bug
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-  🗑️ Remove feature or internal logic
- removes deprecated `DeadLetterChannel`
https://github.com/knative/eventing/blob/main/pkg/apis/duck/v1/channelable_types.go#L70-L74

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- [x] **At least 80% unit test coverage**
- [x] **E2E tests** for any new behavior
- [ ] **Docs PR** for any user-facing impact
- [ ] **Spec PR** for any new API feature
- [ ] **Conformance test** for any change to the spec

**Release Note**

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```release-note
removes deprecated DeadLetterChannel in favor of DeliveryStatus
```


**Docs**

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---------

Signed-off-by: Vishal Choudhary <sendtovishalchoudhary@gmail.com>

docs/eventing-api.md

@@ -282,22 +282,6 @@ DeliveryStatus
 resolved delivery options.</p>
 </td>
 </tr>
-<tr>
-<td>
-<code>deadLetterChannel</code><br/>
-<em>
-<a href="https://pkg.go.dev/knative.dev/pkg/apis/duck/v1#KReference">
-knative.dev/pkg/apis/duck/v1.KReference
-</a>
-</em>
-</td>
-<td>
-<em>(Optional)</em>
-<p>DeadLetterChannel is a KReference and is set by the channel when it supports native error handling via a channel
-Failed messages are delivered here.
-Deprecated in favor of DeliveryStatus, to be removed September 2022.</p>
-</td>
-</tr>
 </tbody>
 </table>
 <h3 id="duck.knative.dev/v1.DeliverySpec">DeliverySpec

pkg/apis/duck/v1/channelable_types.go

@@ -67,11 +67,6 @@ type ChannelableStatus struct {
 	// resolved delivery options.
 	// +optional
 	DeliveryStatus `json:",inline"`
-	// DeadLetterChannel is a KReference and is set by the channel when it supports native error handling via a channel
-	// Failed messages are delivered here.
-	// Deprecated in favor of DeliveryStatus, to be removed September 2022.
-	// +optional
-	DeadLetterChannel *duckv1.KReference `json:"deadLetterChannel,omitempty"`
 }
 
 var (