emacs: Remove `notmuch-search-authors-width' and fix the use of `notmuch-search-result-format' accordingly
The width of the authors field in search output was previously
specified in two places:
- `notmuch-search-authors-width': the limit beyond which the authors
names are truncated,
- `notmuch-search-result-format': the layout of the search results.
Changing the configuration of one of these may have required the user
to know about and adapt the other accordingly. This led to confusion.
Instead, remove `notmuch-search-authors-width' and perform truncation
based on the relevant field in `notmuch-search-result-format'.