x86: Remove unused IOMMU_STRESS Kconfig

Last use of IOMMU_STRESS was removed in commit 29b68415

 ("x86:
amd_iommu: move to drivers/iommu/"). 6 years later the Kconfig entry is
definitely due for removal.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Joerg Roedel <jroedel@suse.de>
Link: https://lkml.kernel.org/r/1516825754-28415-1-git-send-email-clabbe@baylibre.com