fix(typings): fix subscribe overloads

Remove the no-arg overload. If not removed, any Observable will be
compatible with any ObservableInput regardless of type - as T does not
appear in the no-arg overload.

Closes #3052

src/Observable.ts

@@ -73,8 +73,7 @@ export class Observable<T> implements Subscribable<T> {
     return observable;
   }
 
-  subscribe(): Subscription;
-  subscribe(observer: PartialObserver<T>): Subscription;
+  subscribe(observer?: PartialObserver<T>): Subscription;
   subscribe(next?: (value: T) => void, error?: (error: any) => void, complete?: () => void): Subscription;
   /**
    * Invokes an execution of an Observable and registers Observer handlers for notifications it will emit.