Delete References to unmounted items & correctly unmount format answer examples.

Summary:
There is no need to keep around refences to unmounted PerseusTranslationComponents,
so I now set them to null when unmounting.  However I noticed that one component
still stuck around after this change.  It turns out the format answer component
was never properly unmounted, because it was removed by jquery.  React was spitting
out a warning about jquery removing a react component.  It still does it when you
hover over the format answer qtip at some point, but the warning is no longer true.
We do unmount it, but since the qtip created an exact copy of the html, React thinks
it owns it when it actually doesn't.

Test Plan:
make all
Go to http://translate.khanacademy.dev/preview/content/items?exercises=functions_1#x4e7134ce
Click between different items, making sure to occassionaly hover over the tooltip.
In the console type:
window.PerseusTranslationComponents
and see that all the components are now null except for the currently loaded assessment items
components.

Reviewers: jack

Reviewed By: jack

Subscribers: alex

Differential Revision: http://phabricator.khanacademy.org/D9389