Remove broken test_getClientRects.html

This broke in ab0d620 and in trying to fix it, it's become clear that I don't
understand the intended behavior of getClientRects().  In fact I only see one
rect in Firefox even when all three <div>s have drawn borders, which confuses
me.  Let's remove the test for now and someone who understands what's going on
(which could be future me!) will re-add it.
auto merge of #1121 : kmcallister/servo/client-rect, r=metajack

This broke in ab0d620 and in trying to fix it, it's become clear that I don't understand the intended behavior of `getClientRects()`.  In fact I only see one rect in Firefox even when all three `<div>`s have drawn borders, which confuses me compared to [the spec](http://www.w3.org/TR/cssom-view/#the-getclientrects-and-getboundingclientrect-methods).  Let's remove the test for now and someone who understands what's going on (which could be future me!) will re-add it.