Adventure: stop signing my own posts

The source tag credits an outlet Pete is relaying (`ars technica`). On
his own reporting it rendered as a trailing `pete` under a message he
already sent - him signing his own name. Drop Source on adventure posts
and omit the tag line entirely when there's nothing to credit; RSS posts
keep theirs.

internal/web/adventure_digest.go

@@ -94,7 +94,6 @@ func (s *Server) postDailyDigest(now time.Time) {
 		Headline:   headline,
 		Lede:       lede,
 		ArticleURL: s.digestURL(date),
-		Source:     advSource,
 		Channel:    s.adv.Channel,
 	})