Maintainers.txt: Remove me from maintainers of UefiPayloadPkg,ShellPkg

Signed-off-by: Ray Ni <ray.ni@intel.com>
Cc: Guo Dong <guo.dong@intel.com>
Cc: Sean Rhodes <sean@starlabs.systems>
Cc: James Lu <james.lu@intel.com>
Reviewed-by: Gua Guo <gua.guo@intel.com>
Reviewed-by: Zhichao Gao <zhichao.gao@intel.com>
Reviewed-by: Gua Guo <gua.guo@intel.com>