UefiCpuPkg/MtrrLib: Remove unnecessary API MtrrSetFixedMtrr()

UefiCpuPkg/MtrrLib: Remove unnecessary API MtrrSetFixedMtrr()

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

MtrrSetFixedMtrr() sets all the fixed 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 MtrrSetFixedMtrr()
to simplify the MtrrLib API.

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

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>