It is necessary to have different image sizes because a single image size cannot be adjusted to display as a thumbnail, medium size, and large image. Three images are created by WordPress default option and others are generated by theme. we need to display different image sizes in different places that’s why it’s required and cannot be optional.
If you set WordPress created media sizes to 0, your future gallery display may not work properly. I don’t recommend you to do that as it may cause problems in the future.
If you want to do it anyway this article may help you
https://perishablepress.com/disable-wordpress-generated-images/