notmuch.el: Don't document mouse actions in notmuch-help.

The concept behind direct manipulation with mouse clicks is that
documentation shouldn't be necessary, (though my original motivation
here was simply that "<mouse-1>" was exceeding my TAB width.

This does cause a blank line to be added for the mouse binding. This
isn't directly desired, but as long as it's there we put it at a
natural place for a separator.