GraphQL: Remove channel stats from the API (#21377)

Channel Stats isn't optimized with dataloaders.

And it's a heavy call, therefore clients
can mistakenly call this with multiple channels
and bring down the server.

Since there is no advantage compared to a REST API,
let's just remove it and keep things safe.

```release-note
NONE
```

api4/schema.graphqls

@@ -62,7 +62,6 @@ type Channel {
 	totalMsgCount: Float!
 	totalMsgCountRoot: Float!
 	lastRootPostAt: Float!
-	stats: ChannelStats
 	extraUpdateAt: Float!
 	props: StringInterface!
 	policyId: String
@@ -220,10 +219,3 @@ type Session {
 	props: StringMap!
 	local: Boolean!
 }
-
-type ChannelStats {
-	channelId: String!
-	memberCount: Float!
-	guestCount: Float!
-	pinnedPostCount: Float!
-}