bug#35370: notmuch-emacs: avoiding deprecated message-default-charset

[Eli Zaretskii]

> From: Daniel Kahn Gillmor <dkg at fifthhorseman.net>
> Date: Sun, 21 Apr 2019 21:55:38 -0400
> Cc: david at tethera.net, notmuch at notmuchmail.org, 35370 at debbugs.gnu.org
> 
> Do you have any recommendation for how an external emacs module (which
> aims to work with at emacs 25 as well at least) should handle this?

Just remove any uses of it.  Why are you using it now?