How do delete just RAW in RAW+JPEG pair?

The method usually recommended is to Export the pair, delete the pair from Photos, then Import just the JPEG.

The handling of RAW and JPEG pairs has been changed in Catalina. It is now easier to switch between RAW and JPEG Originals, but a bit buggy: Changes to the handling of RAW & JPEG pairs… - Apple Community

But there is still no tool in Photos to separate the RAW and JPEG pairs and probably never will be. Photos has been designed to support a lossless workflow, where we always can revert to the original of an image. To get rid of the originals, we have to delete a photo completely.

If you do not want to have the RAW files in your Photos Library, separate the pairs, before you import them into Photos and only import the RAWs or JPEGs, whichever you want to work with. Once they are in Photos, you have to use the procedure described by Marc to separate them.

I always take 2 photos on my DSLR, a RAW-file and a JPEG-file. I used to import all of them into iPhoto. Then I checked them, threw away a lot, kept some JPEGs just for fun (throwing away the RAWs) and kept interesting photos as a RAW-file, which I then edited in LR or PS (throwing away the JPEGs).

I can’t do that any more, which is disastrous. 

Sometimes I take 300 to 400 photos a day. I can’t separate 400 pairs before I import them, because I don’t know from my camera display how a photo really looks like and whether I want to keep a big file or a small file. Do I have to import 400 photos, check them, export them, separate them, delete one of each pair (remembering, whether I wanted to keep the big or the small one) and import the the other ones second time? That's crazy!

Can't photos work lossless on two different files?

I contacted the support (in Germany) before I read this post. They suggested, that I should trick PHOTOS and import my photos from the desk, not within the App, because PHOTOS would then treat the files as two different documents - what they are indeed. On my card is one file with the name IMG_1290.CR2, (28 MB) and another named IMG_1290.JPG (only 3 MB)). It didn't work.

What can I do ???