x86: clean up amd_iommu documentation

amd_iommu=off was replaced with a common parameter, iommu=off.
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: joerg.roedel@amd.com
Cc: iommu@lists.linux-foundation.org
Cc: bhavna.sarathy@amd.com
Cc: robert.richter@amd.com
Cc: rjw@sisk.pl
Signed-off-by: Ingo Molnar <mingo@elte.hu>

Documentation/kernel-parameters.txt

@@ -274,7 +274,6 @@ and is between 256 and 4096 characters. It is defined in the file
 	amd_iommu=	[HW,X86-84]
 			Pass parameters to the AMD IOMMU driver in the system.
 			Possible values are:
-			off     - disable the driver for AMD IOMMU
 			isolate - enable device isolation (each device, as far
 			          as possible, will get its own protection
 			          domain)