UefiCpuPkg/MtrrLib: Remove unnecessary API MtrrSetVariableMtrr()

UefiCpuPkg/MtrrLib: Remove unnecessary API MtrrSetVariableMtrr()

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2849

MtrrSetVariableMtrr() sets all the variable MTRR settings.
But in fact MtrrSetAllMtrrs() is always used by callers to set all
MTRR settings including the fixed and variable ones.

The patch removes the unnecessary API MtrrSetVariableMtrr() to
simplify the MtrrLib API.

There is no code in edk2 and edk2-platforms repo that calls
MtrrSetVariableMtrr().

Signed-off-by: Ray Ni <ray.ni@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Cc: Rahul Kumar <rahul1.kumar@intel.com>