Remove check/error of childView of CompositeView

CompositeView will always default to itself which is never falsey.  This error has never been thrown at least back to Oct 2012.

https://github.com/marionettejs/backbone.marionette/blob/f955f271f7323ba5f5d708fc9f6be568af22949d/src/marionette.compositeview.js#L29