Remove redunant a11y option setters to default values

[zepumph]

Now with #748 we have default labels/description DOM elements, so often we can simplify code. Like:

new Node( {
 tagName: 'input'
 labelTagName: 'p',
 labelContent: 'This great label for input'
 descriptionTagName: 'p',
 descriptionContent: 'This is a description for the input',
 containerTagName: 'div'
});

would just be

new Node( {
  tagName: 'input'
  labelContent: 'This great label for input'
  descriptionContent: 'This is a description for the input',
 });

Marking for chip away, and tagging for a11 dev meeting.

[zepumph]

Marking as high priority just so we can talk this week about it and all be on the same page.

[jessegreenberg]

We talked about this at an a11y dev meeting, so we are all aware. Thanks for bringing it up @zepumph. We will be revisiting lots of a11y code as we progress with issues like #753. Closing.