refactor(cli)!: remove the 'commands' command (#1462)

This change removes the 'commands' command. This command historically
printed out all of the commands, optionally with help text. This is
superfluous, as we ship a manpage for each of the common shells, have
markdown documentation available in the repository, and provide help
text via the `--help` flag.

BREAKING-CHANGE: The `commands` command has been removed. There is no
replacement
 planned. Users are encouraged to run `--help`, view the markdown
 documentation available online, or run `man git-bug` to view the
 manpage in your terminal.
Change-Id: I8bbfb03c03c820ec0b56549e59ab76826c45b9cc