Consider not specifying db-path in configuration template

[LucasFA]

Is your feature request related to a problem? Please describe.
I have a custom path for CARGO_HOME. Running cargo deny init defaults to including in the configuration the path to the database ~/.cargo/advisory-db. In my case, that creates an unnecessary ~/.cargo folder in my HOME directory.

Describe the solution you'd like
I tested just commenting out the offending line and it seemed to default to my CARGO_HOME, exactly as I want.

Describe alternatives you've considered

• Relying on the existence of the CARGO_HOME environment variable at the time of cargo deny init would probably lead to stale configuration, as now.
• Documenting this fact in the README probably helps warn some - probably not all - users, but seems unnecessary to add to the length of text.

Additional context
Somewhat related: #302. This change would bring the default (aka, without deny.toml) behaviour to the deny.toml default template.