emacs: Truncate lines and do not enable visual-line-mode in notmuch-show buffers.

Enable the truncation of lines in `notmuch-show-mode' to avoid visual
noise caused by the wrapping of the header lines.

Don't enable `visual-line-mode' because it disables line truncation.

The benefits of `visual-line-mode' were that it wrapped long lines
in received messages. With `notmuch-wash-wrap-long-lines' now default
behaviour, this is no longer required.