aws: revert change to support instance types > 255 cpu cores in xenial

[Impact]

In 1913739 we applied a fix to properly detect the right amount of cores in AWS large instance types (> 255). However, this fix seems to work properly only with kernels > 4.4. Even if it doesn't seem to introduce any failure or regression, it is probably just safer to revert this commit in the Xenial kernel, and keep it on the other kernels (> 4.4).

[Test plan]

Boot an instance type with > 255 cpus and check the amount of cores detected.

[Fix]

Revert commit:
 561ef22a7d396289a1c5a1d18057ccdf9bb59826 ("iommu/vt-d: Gracefully handle DMAR units with no supported address widths")

[Regression potential]

Reverting the commit simply restores the old behavior, so we won't be able to detect the right amount of CPUs in large instances, but we are sure that we won't also introduce any potential failure or regressions by restoring the previous behavior. And this is affecting only the Xenial kernel, that is approaching to EOL.