mailutils: use system-sendmail instead of sendmailPath

system-sendmail allows all sendmail's to be auto-detected, including on
non-NixOS systems. This is, to me, a better UX than having to manually
override the sendmailPath argument.

In exchange, it is a breach of retro-compatibility. Given right now I
can't see any uses for sendmailPath other than what is supported by
system-sendmail, I didn't keep it, but it'd be possible to allow
sendmailPath to override the choice of sendmail from system-sendmail.