MdeModulePkg/DxeHttpLib: Use BaseLib api AsciiCharToUpper

MdeModulePkg/DxeHttpLib: Use BaseLib api AsciiCharToUpper

Substitute HttpIoCharToUpper with AsciiCharToUpper which is a public function
with the same effect. And remove the implement of HttpIoCharToUpper.
https://bugzilla.tianocore.org/show_bug.cgi?id=1369

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Shenglei Zhang <shenglei.zhang@intel.com>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>