clean up code that deals with EnabledNode options in Slider

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In d640f5c for #257, we decided that options to EnabledComponent should not be mutually exclusive, because subclasses (and other clients) have a legitimate need to specify defaults.

@zepumph pointed out that this bit of code in Slider.js should therefore be cleaned up:

    // Extra logic to prevent providing two conflicting options to EnabledNode
    if ( !options.enabledProperty ) {
      options = merge( {

        // EnabledNode
        enabledPropertyOptions: {
          phetioFeatured: true
        }
      }, options );
    }

[pixelzoom]

Cleaned up in the above commit. @zepumph please review.