You can filter for the raw files. If you can't see the library filter above all the thumbnails press the backslash key.
Choose text and in the search box type the extension for the raw files e.g. NEF, CR2, ARW etc. You can then select all the results (Ctrl+A) and delete or remove. Removing lets you clear the images from the catalog but keep the files in the folder on your hard drive, unless you confirm you want to delete them permanently to the recycle bin.