x86, mtrr: remove mtrr MSRs double declaration

Removed MTRR MSR from mtrr/mtrr.h as these are already declared in
msr-index.h and nobody is using them:
 MTRRfix16K_A0000_MSR
 MTRRfix4K_C8000_MSR
 MTRRfix4K_D0000_MSR
 MTRRfix4K_D8000_MSR
 MTRRfix4K_E0000_MSR
 MTRRfix4K_E8000_MSR
 MTRRfix4K_F0000_MSR
 MTRRfix4K_F8000_MSR

Use standard msr-index.h's MSR declaration and no need to declare again

[ Impact: cleanup, no object code change ]

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>