Remove list structure on Mass Controls

[terracoda]

In light of phetsims/ohms-law#148 (comment), it is likely best to remove the list structure around the Mass controls.

Hopefully, this is quite easy to do.
@zepumph, I'd keep the aria-labelledby on the parent div.

Possible pseudo-code

<h3 id="mass-heading">Mass Controls</h3>
<div aria-labelledby="mass-heading">
<Mass 1 slider/number spinner>
<Mass 2 slider/number spinner>
</div>

[zepumph]

I ended up replacing the UL with a div, and the LI with a div. The resulting output has more divs, but keeps the aria-valuetext in the a11y view more inline:

When the LI tags are just removed, we get this:

I can do either one, but just committed the one with an extra nested 'div' please review.

[terracoda]

@zepumph, the goup of mass controls sounds great on VoiceOver! Closing this issue.