MdeMoudlePkg/CapsulePei: No need to remain space for null-terminate

MdeMoudlePkg/CapsulePei: No need to remain space for null-terminate

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1853

UnicodeValueToStringS would remain two bytes for the first
parameter to set the null- terminate. So remove this change
in Mu.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Michael Turner <Michael.Turner@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Signed-off-by: Zhichao gao <zhichao.gao@intel.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>