Remove server subcommand from --help

[lox]

Some notes:

The "proxy" server starts up in the background and binds to 169.254.169.254:80, which is what the aws tooling connects to. It needs root permissions to create the iface and bind to a port < 1024. This is what is triggered by aws-vault server. It's meant to be pretty useless on it's own, and simply proxies requests to localhost:9099 if it exists.

The "credential" server is started at the beginning of an aws-vault exec --server invocation and it only runs for the duration of that command. It's the one with the keys to the kingdom, so it binds to localhost:9099.
#174 (comment)

Closes #207.

[avram]

Is there a reason this PR is marked WIP? looks like the right thing to do-- and I've been confused many times by the tempting command that is in aws-vault --help that I'm not supposed to run.

[lox]

@avram the docs are still in progress, but this is probably a useful change on it's own.

[avram]

Right-- the actual docs could be improved as well but removing it from aws-vault --help is a big improvement as that is the only place someone would see that this subcommand even exists.