Why are there so many useless masks when I drag and drop photos from LRT into Lightroom?
How to cancel these masks?

Please don't, otherwise LRTimelapse won't work as expected. Those masks get initialized by LRTimelapse. The Masks marked as "for internal use" are needed internally by LRTimelapse do do Deflicker, Holy Grail Wizard etc. And the other 6 masks are Masks that you can use to keyframe masks animations.
If you don't do that, just ignore them.
Check out my Tutorials, it's all explained there: Https://lrtimelapse.com/tutorial/

Thanks for your answer!
I will read the Tutorials carefully to learn more