Drop libidn11-dev build-dep

Simon Dezielrequested to merge
sdeziel-guest/msmtp:libidn into unstable

I checked the configure output of various buildd arch like [1] to conclude that IDN support from an external library was not needed. This change was (lightly) tested with a IDN name as "host" and it worked.

While in there I did 2 minor changes to debian/control.

1: "foo":https://buildd.debian.org/status/fetch.php?pkg=msmtp&arch=i386&ver=1.8.2-1&stamp=1547680876&raw=0

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