Deleting all JPEG with a RAW counterpart

Filtering on Raw will hide any separate images that are JPG. Sometimes (depending on setting active at time of import) a Raw+JPG pair from the camera will show as a single Raw+JPG image, in that case you cannot interact with the JPG using LrC at all, that remains a silent and inactive partner. I suspect the issue here is that the JPGs are showing as separate images, and you don't want those present in the Catalog - but you do want to keep those JPGs that were shot without any Raw alongside.

There is no simple way I know of to directly distinguish JPGs that have a related Raw alongside, from JPGs that do not. [edit: no way provided in standard app. Bob's post below leads to a nice little script from John Beardsworth, here which I haven't tried but looks just the ticket - it works by checking which Raws have got related JPGs, which is a far more sensible idea!!]

But in general, we don't continually switch the camera between Raw+JPG / JPG mode from shot to shot. That change of mode happens less frequently: per whole occasion, perhaps. So by viewing image thumbnails in date-taken chronological order in Grid view, one can quickly identify for a whole batch of photos which way the camera was set when taken. And that helps differentiate wanted from unwanted JPGs. You can then mark the unwanted ones as rejects (with X key). Then backup the Catalog and backup all the imported files. Then review and bulk delete the rejects.

Personally I resolve this more radically by setting just [Raw only] / [JPG only] in the camera. To me, Raw+JPG is not the 'best of both worlds' - but rather, a compromised third-best. The presence of each undermines the other creatively IMO, plus optimal exposure etc strategy must be a little different for each. Best to just decide, commit and accept.