[Quantal] Revert/drop EC2 performance patch

[Stefan Bader]

As a result of some hallway discussion I found that Amazon
actually does not really need this patch (anymore?). And
looking at the code a bit more this would only make a difference
if the blkback would pass the number of buffers via sysfs (which
looking at an instance it does not). So this likely never made
it into the dom0 of EC2 and we can drop it (or rebase away).

I do not think it is worth to SRU that back into previous releases
as there is zero change in the drivers real behaviour.

-Stefan