ArmPkg/PlatformBootManagerLib: hide UEFI Shell as a regular boot option

ArmPkg/PlatformBootManagerLib: hide UEFI Shell as a regular boot option

Without ConnectAll() being called on the boot path, the UEFI shell will
be entered with no block devices or anything else connected, and so for
the novice user, this is not a very accommodating environment. Now that
we have made the UiApp the last resort on boot failure, and made the
UEFI Shell accessible directly via the 's' hotkey if you really need
it, let's hide it as an ordinary boot option.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>