Delete only JPEG's

Or a handy Script can do the job, from John Beardsworth-

JPEGs sidecar killer – Lightroom Solutions

The simplest way is in the Library Module, grid view, sort by file extension. All the .jpg files will be grouped together. Select them all and delete from there.

One other way to do it is in Lightroom Classic, right click on one of the images and select Show in Finder/Explorer. That will put you in the folder where the images are located on your drive. In Finder or Win Explorer, sort by type. Then select all the .jpg images and delete them. 

Back in Lightroom Classic select the relevant folder, right click and select synchronize folder. You will then get a screen to remove the missing .jpgs from Lightroom Classic.

You need to do this before exporting any jpgs into the original folder, otherwise, they will be deleted too.