If neither neomutt.sendMailCommand nor passCmd are set, leave
configuration for sending emails unset. This will leave neomutt to
manage sending emails itself, which matches the existing description of
the `accounts.email.accounts.<name>.neomutt.sendMailCommand`
configuration option.
This also removes the assertion added in bd680a8c (neomutt: allow
default email sending behaviour, 2025-07-12), since it is now valid to
have neither passCmd nor neomutt.sendMailCommand on an account.