pubsub: remove experimental notice from dead letter queue fields

Fixes #1945

Change-Id: Iec3b4e64f5d7f9e04090cab2a2ec142a590fa191
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/55210
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Cody Oss <codyoss@google.com>
Reviewed-by: Seth Hollyman <shollyman@google.com>
diff --git a/pubsub/message.go b/pubsub/message.go
index ed76a66..13dfbc6 100644
--- a/pubsub/message.go
+++ b/pubsub/message.go
@@ -54,10 +54,6 @@
 	// If dead lettering is enabled, this will be set on all attempts, starting
 	// with value 1. Otherwise, the value will be nil.
 	// This field is read-only.
-	//
-	// It is EXPERIMENTAL and a part of a closed alpha that may not be
-	// accessible to all users. This field is subject to change or removal
-	// without notice.
 	DeliveryAttempt *int
 
 	// size is the approximate size of the message's data and attributes.
diff --git a/pubsub/subscription.go b/pubsub/subscription.go
index 95666a7..cc09209 100644
--- a/pubsub/subscription.go
+++ b/pubsub/subscription.go
@@ -232,10 +232,6 @@
 
 	// DeadLetterPolicy specifies the conditions for dead lettering messages in
 	// a subscription. If not set, dead lettering is disabled.
-	//
-	// It is EXPERIMENTAL and a part of a closed alpha that may not be
-	// accessible to all users. This field is subject to change or removal
-	// without notice.
 	DeadLetterPolicy *DeadLetterPolicy
 }
 
@@ -320,9 +316,6 @@
 
 // DeadLetterPolicy specifies the conditions for dead lettering messages in
 // a subscription.
-//
-// It is EXPERIMENTAL and a part of a closed alpha that may not be
-// accessible to all users.
 type DeadLetterPolicy struct {
 	DeadLetterTopic     string
 	MaxDeliveryAttempts int
@@ -485,9 +478,6 @@
 
 	// If non-nil, DeadLetterPolicy is changed. To remove dead lettering from
 	// a subscription, use the zero value for this struct.
-	//
-	// It is EXPERIMENTAL and a part of a closed alpha that may not be
-	// accessible to all users.
 	DeadLetterPolicy *DeadLetterPolicy
 
 	// If non-nil, the current set of labels is completely