Hide deprecated subcommands from debusine-client's --help output

When using bash-completion with debusine-client, it actually runs debusine --help to gather possible subcommands. The client helpfully prints all possible commands including the deprecated ones. It would be helpful for retraining the finger memory if completion only provided non-deprecated subcommands. This effectively translates to --help displaying only non-deprecated subcommands.

There is a downside as well. If a user encounters a deprecated invocation and tries to understand it using --help, they no longer can. I suggest that at this phase of development, accelerating deprecations is more beneficial still.