Polaris-react: [RangeSlider] remove implementation of accessibility inputs

[danrosenthal]

784 implements the DualThumb range slider with text inputs for accessibility purposes. These should be removed in favor of documentation that describes or directly illustrates the use of alternative inputs for accessibility. This could either be illustrated in a second example, or described in the Accessibility section added in @dpersing's accessibility documentation work.

In addition to removing any markup and handlers for these elements, we will also want to remove the prop specified here.

|accessibility inputs shown in red|
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[dpersing]

documentation that describes or directly illustrates the use of alternative inputs for accessibility.

@danrosenthal I'd strongly favor:

• Documentation that describes the accessibility differences between the single- and dual-thumb sliders out of the box (I can work on that today/this week as part of the style guide updates)
• A best practice Playground/Storybook example showing the dual-thumb slider with text/number inputs.

[danrosenthal]

@dpersing I'm good with that!

[solonaarmstrong-zz]

@dpersing FYI, Liz and I have a WIP PR.