Remove unnecessary protocol in Combine (#3650)

### Goals :soccer:
<!-- List the high-level objectives of this pull request. -->
<!-- Include any relevant context. -->

Subscription is Cancellable. Remove unnecessary Cancellable adoption in
Inner of Combine.


### Testing Details :mag:
<!-- Describe what tests you've added for your changes. -->
None

Source/Combine.swift

@@ -87,7 +87,7 @@ public struct DataResponsePublisher<Value>: Publisher {
                                                downstream: subscriber))
     }
 
-    private final class Inner<Downstream: Subscriber>: Subscription, Cancellable
+    private final class Inner<Downstream: Subscriber>: Subscription
         where Downstream.Input == Output {
         typealias Failure = Downstream.Failure
 
@@ -308,7 +308,7 @@ public struct DataStreamPublisher<Value>: Publisher {
                                                downstream: subscriber))
     }
 
-    private final class Inner<Downstream: Subscriber>: Subscription, Cancellable
+    private final class Inner<Downstream: Subscriber>: Subscription
         where Downstream.Input == Output {
         typealias Failure = Downstream.Failure
 
@@ -458,7 +458,7 @@ public struct DownloadResponsePublisher<Value>: Publisher {
                                                downstream: subscriber))
     }
 
-    private final class Inner<Downstream: Subscriber>: Subscription, Cancellable
+    private final class Inner<Downstream: Subscriber>: Subscription
         where Downstream.Input == Output {
         typealias Failure = Downstream.Failure