Have you clearly marked your good photos, ex. with 5 stars or with the green color marker?
If yes you can filter your catalog after this or you set a filter that show all photos except the good ones.
If not I see no other chance to go manually through your catalog and marked the appropriate good photos or delete the others.
Philosophy from @dj_paige (all rights reserved)
The Lightroom catalog should contain ALL of the photos you are going to keep. That's ALL, every single one, 100% of the photos you will keep, no exceptions. If you decide that a photo is so poor or unusable or not likely you'll ever want to look at it or use it again, then delete it from Lightroom and from your hard disk.
Why do I say this?
Lightroom is the tool by which you will find the photos (all 100% of the keepers) in the future. Any system of organization that relies on Software A to find some photos and Software B (which could be your operating system) to find other photos is going to be confusing and ineffective in the long run. If all of your photos are in Software A, you know immediately where to find them.
So, to discuss your original problem statement: the solution is to not get rid of the bad photos in Lightroom and keep them elsewhere, the solution is to ORGANIZE all of your photos in Lightroom. That makes them easy to find. If you just want to "hide" some of the really bad ones that you are keeping, there are many ways to do this: one such way is to assign the bad ones a 1-star rating, and then create a filter that doesn't show the 1-start ratings. A better way is to provide keywords and other metadata in Lightroom for all of your keepers, so that you can find any photo at any point in the future in Lightroom.