perf: remove superfluous AsObservable calls in ReactiveCommand

[kentcb]

Do you want to request a feature or report a bug?

Code issue.

What is the current behavior?

ReactiveCommand includes several properties that return observables, but calls AsObservable on those return values.

What is the expected behavior?

There is no need to call AsObservable - it only adds to the performance cost by creating another object on top of the already-usable observable.

What is the motivation / use case for changing the behavior?

Better code and performance.

Which versions of ReactiveUI, and which platform / OS are affected by this issue? Did this work in previous versions of ReativeUI? Please also test with the latest stable and snapshot (http://docs.reactiveui.net/en/contributing/snapshot/index.html) versions.

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