x86/cpu/AMD: Remove now unused definition of MFENCE_RDTSC feature

x86/cpu/AMD: Remove now unused definition of MFENCE_RDTSC feature

With the switch to using LFENCE_RDTSC on AMD platforms there is no longer
a need for the MFENCE_RDTSC feature.  Remove its usage and definition.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Orabug: 27340445
CVE: CVE-2017-5753

Signed-off-by: Chuck Anderson <chuck.anderson@oracle.com>
Conflicts:
	Patch refers to arch/x86/include/asm/cpufeatures.h
	Code base has   arch/x86/include/asm/cpufeature.h
	Patch references X86_FEATURE_MFENCE_RDTSC in arch/x86/include/asm/msr.h
	Code base references it in:
		arch/x86/include/asm/barrier.h
		arch/x86/um/asm/barrier.h

Reviewed-by: John Haxby <john.haxby@oracle.com>
Signed-off-by: Kirtikar Kashyap <kirtikar.kashyap@oracle.com>