If you removed the photos from Lightroom, I don't think there is any way to go back into Lightroom and remove the watermark (other that re-doing the entire process).
My advice, in the future, is that once you import photos into Lightroom, you do not ever remove them from Lightroom.
I know, but I had other photos to import (another mariage), and if I don't delete the first 600, it's too much for my computer. I had hope to find a solution, because when I imported the 600 photos with the watermark (after my mistake), and I opened it for the first time, each picture loaded for a few time (1 or 2 seconds) without the watermark that just appeared at the end. Si I thought that the watermark is not fixed yet in the picture, and there's still a way to remove it.
I know, but I had other photos to import (another mariage), and if I don't delete the first 600, it's too much for my computer.
Honestly, I think this is incorrect. Maybe you mean ... it's too much for my hard disk ... but that's different, that's not what you said. But if that's what you mean, don't you have additional hard disks where you can store your original photos?
If your photos are no longer imported into Lightroom, if you have removed them from Lightroom, and all you have are the exported photos, there is no easy way to remove the watermark.
I mean that if you have 5000 photos on lightroom, it's slow to open lightroom and it takes RAM, but that's not very important. It's strange because, in the files, when you look at the small picture ("miniatures" in french), there is no watermark, the watermarks just appears when I open the picture in full screen. That makes me thing that watermarks are an additional data that is not stamped directly on the picture... But if there is no way, too bad for me ! Thanks for answer anyway.