always just send IP instead of hostname in SMTP EHLO message

Clients are very often behind NATs, which makes the hostname in
HELO/EHLO messages virtually useless these days. Attempting to figure
out a hostname we could use also led to issues with some strict Postfix
configurations (see https://github.com/k9mail/k-9/issues/3387). This
commit changes our behavior to simply send the local IP always, getting
rid of this metadata.

Fixes #3387