Only expose RTC backend identity debug info where we actually use it

We only ever inspect the RTC backend identity of user media tiles. So it only needs to be exposed on the user media view model.

src/state/UserMedia.ts

@@ -143,7 +143,6 @@ export class UserMedia {
                     scope,
                     `${this.id}:${key}`,
                     this.userId,
-                    this.rtcBackendIdentity,
                     p,
                     this.encryptionSystem,
                     this.livekitRoom$,