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

db/tx.go

@@ -52,7 +52,6 @@ func (r Result) RowsAffected() (int64, error) {
 }
 
 type Rows struct {
-	tag pgconn.CommandTag
 }
 
 func (r Rows) Close() error {