Fix kmath loading error

Summary:
old perseus didn't require khan-exercise.js when we added requirejs to
khan-exercises, we added khan-exercise.js as a dependency of perseus.
This was overwriting the earlier stub KhanUtil global we had created,
which happened to often be where kline/kvector/etc were defined, since
those were being loaded very early on in the process.

This commit removes khan-exercise.js, since we weren't using anything
from it and would prefer fewer dependencies, all else being equal,
and with it, the imposter KhanUtil. (Well, this "imposter" Mr. KhanUtil
was probably the original KhanUtil, but in this specific perverse case
of identity theft, we had come to love the new Mr. KhanUtil more than
the original, may he ever be fondly remembered <3)

Test Plan:
 * open test.html many times
 * do you see "undefined has no method "equal"?
    * no -> yay
    * yes -> this commit didn't work

Auditors: alpert, justinjaffray