[CommandLine][NFCI] Do not add 'All' to 'RegisteredSubCommands' (#77041)

After #75679, it is no longer necessary to add the `All` pseudo
subcommand to the list of registered subcommands. The change causes the
list to contain only real subcommands, i.e. an unnamed top-level
subcommand and named ones. This simplifies the code a bit by removing
some checks for this special case.