MM-61225: Revert session pooling (#28901)

This originated from https://github.com/mattermost/mattermost/issues/15249.

However, the original idea was discarded https://github.com/mattermost/mattermost/issues/15249#issuecomment-709713065
as being too complicated to implement. Then I had another
idea to implement it just for session objects.

My thinking was that since every single request allocates a new
session struct, it would be good to use a sync.Pool for that.

However, 4 years later, now we know that the primary bottleneck
in app performance comes from websocket event marshalling.
Therefore, while it would be good to do this, it is difficult
to do it correctly (as shown by the numerous racy tests).

Hence, reverting this.

```release-note
NONE
```

server/channels/wsapi/websocket_handler.go

@@ -30,7 +30,6 @@ func (wh webSocketHandler) ServeWebSocket(conn *platform.WebConn, r *model.WebSo
 		return
 	}
 	session, sessionErr := wh.app.GetSession(conn.GetSessionToken())
-	defer wh.app.ReturnSessionToPool(session)
 
 	if sessionErr != nil {
 		mlog.Error(