simplify and fix automailer parser config

fix #19929 (closed)

Tested that:

• ./bin/generate-call-for-trusted-reproducer --version 5.17 doesn't fail
  • setting mailer = "thunderbird" in ~/.config/tails/automailer/mua.toml does what's intended
  • but using --mailer overrides that
• ./bin/automailer.py doesn't fail, and has same behavior as explained above