lint: remove unused items from resource, sync, platform, db

- Remove contentUploadList/Placeholder/Detail from resource/upload.go
- Remove toImageURLs, userURI from resource/communication.go
- Remove responseListUser from resource/user.go
- Remove contentSignin/getSignin from sync/signin.go (re-add contentSignin used elsewhere)
- Delete sync/service-request.go (entirely unused)
- Remove _rowWithID from platform/signal.go
- Remove unused tag field from db/tx.go Rows struct

sync/signin.go

@@ -16,12 +16,6 @@ type contentSignin struct {
 	Next               string
 }
 
-func getSignin(w http.ResponseWriter, r *http.Request) {
-	errorCode := r.URL.Query().Get("error")
-	next := r.URL.Query().Get("next")
-	signin(w, errorCode, next)
-}
-
 func getSignout(w http.ResponseWriter, r *http.Request, user platform.User) {
 	auth.SignoutUser(r, user)
 	http.Redirect(w, r, "/signin", http.StatusFound)