feat: remove <name> <version> header from help text

[sigoden]

The PR removed the <name> <version> header from the generated help text.

Old help text

prog 1.0.0
This is a test CLI

USAGE: prog [OPTIONS] <COMMAND>

New help text

This is a test CLI

USAGE: prog [OPTIONS] <COMMAND>

[pepa65]

This is sad, you're sabotaging your own tool...

[sigoden]

This really depends on personal preference. However, I personally don't think it's necessary to display the command name and version in the first line. Well-known CLI frameworks, such as Rust's clap and Go's cobra, do the same.