qt: fix image response handling

only delete image response on finished

documentation of QQuickImageResponse mentions that a response may only be deleted after QQuickImageResponse::finished signal

quote from QQuickImageResponse::cancel signal

Note: finished() should not be emitted until the response is complete,
regardless of whether or not cancel() was called. If it is called
prematurely, the engine may destroy the response while it is still active
, leading to a crash.

also remove unused classes