Remove the undocumented `help` subcommand.

This is processed by hand in CommandObjectMultiword, and is undiscoverable,
it doesn't work in all cases.  Because it is a bare word, it can't really be
extended w/o introducing the possibility of collisions as well.  If we did
want to do something like this we should add a --help flag to CommandObject.  That
way the feature would be consistent and documented.

Differential Revision: https://reviews.llvm.org/D142067