debian: move vim directory creation to notmuch-vim binary package

This step was missed during the package split of notmuch to notmuch,
notmuch-emacs, and notmuch-vim.

It seems mostly harmless in this case, but it is silly for non-vim
users to have those directories.
(cherry picked from commit 4b5875d81ef80dd7138e178f59ac67f66d5fe0ab)