Maintainers.txt: Remove UEFI Shell Binaries section

The Shell binaries are not generated anymore in each
stable tag release.
So, remove the section.

Reviewed-by: Zhichao Gao <zhichao.gao@intel.com>
Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Reviewed-by: Ard Biesheuvel <ardb+tianocore@kernel.org>
Signed-off-by: Ray Ni <ray.ni@intel.com>