Remove Subscription Wrapper from Observable.subscribe

Performance increase from 7.7m to 8.8m ops/second

Before:

Benchmark                              (size)   Mode   Samples         Mean   Mean error    Units
r.u.PerfTransforms.mapTransformation        1  thrpt         5  7714453.187   362218.633    ops/s
r.u.PerfTransforms.mapTransformation        1  thrpt         5  7527144.767   497816.471    ops/s

After:

Benchmark                              (size)   Mode   Samples         Mean   Mean error    Units
r.u.PerfTransforms.mapTransformation        1  thrpt         5  8853534.333   183494.242    ops/s
r.u.PerfTransforms.mapTransformation        1  thrpt         5  8806958.403   452595.219    ops/s

rxjava-core/src/main/java/rx/Observable.java

@@ -6255,16 +6255,7 @@ public final Subscription subscribe(Subscriber<? super T> subscriber) {
                 subscriber = new SafeSubscriber<T>(subscriber);
             }
             onSubscribeFunction.call(subscriber);
-            final Subscription returnSubscription = hook.onSubscribeReturn(subscriber);
-            // we return it inside a Subscription so it can't be cast back to Subscriber
-            return Subscriptions.create(new Action0() {
-
-                @Override
-                public void call() {
-                    returnSubscription.unsubscribe();
-                }
-
-            });
+            return hook.onSubscribeReturn(subscriber);
         } catch (Throwable e) {
             // special handling for certain Throwable/Error/Exception types
             Exceptions.throwIfFatal(e);