If it was possible to remove the watermark from image then it wouldn't perform it's task - to protect images from use and/or copying by others.
However if you follow this workflow you'll be able to achieve that:
1. Do not apply watermark to full size images
2. Install any plugin that allows regenerating images (for e.g. AJAX Thumbnail Rebuild or Regenerate Thumbnails). Both allow you to regenerate images using an original (full size) image.
3. Then, if your original (full size) image doesn't have a watermark applied to it you may regenerete other image sizes and so remove watermark.