Bug 872497 - Remove problematic "optimization" in NS_QuickSort. r=jlebar

This change allows us to avoid O(n^2) performance when partially sorted data
is given to NS_QuickSort.  We no longer attempt an insertion sort if the input
appears pre-sorted.  This brings the expected performance back to O(n*log(n))
but also eliminates the possibility of O(n) best case performance.  Attempts
to achieve O(n) performance should be the responsibility of callers as they
are in a better position to evaluate the costs/benefit trade-off of looking
for special cases that can be sorted quicker.