UefiCpuPkg/MtrrLib: Remove unnecessary API MtrrGetVariableMtrr()

UefiCpuPkg/MtrrLib: Remove unnecessary API MtrrGetVariableMtrr()

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

MtrrGetVariableMtrr() returns all the variable MTRR settings.
But in fact MtrrGetAllMtrrs() and
MtrrGetMemoryAttributeInVariableMtrr() are used by callers to get the
MTRR settings. The former one returns both the fixed and variable
MTRR settings.

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

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

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>