remove concept of prod and dev assets

instead always do the "prod" thing, but use entr for restarting the
server for all assets

cmd/gonic/main.go

@@ -54,7 +54,6 @@ func main() {
 		db,
 		*musicPath,
 		*listenAddr,
-		"server/assets/",
 	)
 	err = s.SetupAdmin()
 	if err != nil {